What do I need to prepare as a US student to apply to UCL?

For all Unis, if you are a 'near miss', they may still accept you but there are no guarantees.
So make sure that your Insurance choice is a Uni/course you would still want to go to.

applying for history and politics of the americas.
If I didn't meet the AP score requirements, will I still have a chance to get in?

For a "near miss", would they only give conditional offers or also unconditional? Since getting conditional offers are pretty risky due to the intensity and unpredictablility of AP tests.

As above if on results day you are a "near miss" (i.e. got slightly below your offer requirements) they may accept you but are not required to. Very much depends on how near a miss you are and how popular/competitive the course you're applying to is. If you're not able to meet the requirements (e.g. you aren't taking enough AP classes to start with) then you will be immediately rejected.

For undergraduate study in the UK you will always get a conditional offer unless you are applying as a post-qualification applicant (apart from a very few visual/performing arts courses which give unconditional offers on the basis of audition/portfolio). The condition will be to achieve the stipulated grades in the offer by a given date in the summer (usually late July or early August I think). The "near miss" process happens after you get your final results, upload them to Portico in the case of UCL, and you have not quite made the requirements of the offer.

If you do achieve the requirements of the offer though the university are required to accept you, if you wish to matriculate (although as an international student the offer may also include conditions of e.g. financial declarations etc).

Thanks for the info! However I still have one more question:
If I already meet the grades of AAB required for the "History and politics of the Americas" course at the time I am applying, will UCL just give me a straight unconditional offer? Or do they want higher grades such as AAA?

If you already meet the requirements then they may give you an unconditional offer - they could give you a conditional offer based on the remaining classes you're taking though. I would suspect you'd either get an unconditional offer or rejection though. Note even if you get an unconditional offer, there is the soft condition that you will usually need to upload confirmation of your results to Portico and as noted, you might also have to do some kind of financial declaration.

You can apply up to 5 courses in total. Sometimes people apply to different courses at the same University but the process is about applying for a subject rather than a University so generally people apply for 5 different universities.

Got you! Also it is well established that you can apply for 5 universities in the UK. Can I apply for multiple courses in one university and still apply for a maximum of 5 universities via UCAS? Or the rule is that you can only apply for 5 universities via UCAS, one course for each university?

It's 5 courses, not universities. You can apply to 5 courses at 1 university, 1 course each at 5 universities, or a few at a single university and individual courses at others - up to a total of 5. Also worth noting, you can only apply to one of Oxford or Cambridge (unless an Organ Scholar, although I think they are removing that, or maybe have already, or applying to graduate entry medicine), and a maximum of 4 vet med/dentistry/medicine courses.

It's generally not advisable to apply to multiple courses at a single university as usually either the courses are similar enough that your application would be reviewed by the same people and if you were a suitable candidate for one, you would be for the other (and if not, vice versa - so essentially there is no benefit to applying to multiple courses at the same uni in that case) and often unis will make offers for similar courses if you are just "edged out" by competition for a popular course.

It's also not usually advisable to apply to substantially different courses (either at the same or different unis) as you only get one personal statement for all five of your choices. Trying to split your PS between two different courses usually will just dilute it too much for either course and make it more like you won't get an offer from either course. The exception is for vet med/dentistry/medicine, as you can only apply to a maximum of 4, unis often will accept a medicine personal statement for a similar bioscience course (e.g. biomedical/bioveterinary/biodental sciences, and similarly named courses), although this isn't always the case (they usually will state if they do or don't).

It is often reasonable to apply to several closely related but not identical courses - e.g. ancient languages plus classics, or biological sciences plus ecology and conservation. Also for courses that are rare/unique (e.g. economic history, land economy, human sciences) it's often anticipated the personal statement may be a bit more focused on a more general analogue of those subjects (e.g. history, economics, or anthropology/biosciences, respectively, for those examples).

Thank you for this long chain of information! Also is it possible that I can change my course of degree after I got into an university if I am not satisfied with the course I applied for? Such as in the US after one year students can transfer to a different major. Is that an option likewise in the UK? Thanks.

Sometimes but it's not that straightforward. If the courses are largely the same (e.g. transferring from an "X with Y" course to just the "X" version), or have a shared first year (for example a number of life sciences courses at KCL I think have a shared first year), then it's often possible to internally switch into the second year of the new course directly.

If they are quite different you will normally need to start in first year of the new course, and there's no guarantee you could switch internally - they may advise you will need to reapply through UCAS. You will also often be expected to meet the standard entry criteria for the new course even for an internal transfer; it's usually designed to prevent people from "backdooring" from an unpopular course onto a very popular and competitive one that they don't meet the requirements for normally.

Also some courses do not accept internal (or external) transfers as a matter of course - most clinical courses for example.

Thanks for the clarification! Really helpful indeed!