Marking a UCAS tab "complete"Watch this thread
Yes, I'm sure that you can go back and edit the tab later if it's completed as I have done it before the only time anything can't be edited is when you pay for the application and send it off but that's after everthing
When you mark a section as complete you can edit it up until the point you completely submit to UCAS and pay your fee. I believe if you or UCAS do notice an error it can be changed after. So just make sure it as correct as you believe it to be at the time of submission. The places you apply to will most likely ask to see evidence of your qualifications, either directly from you or through an institution if you're applying through school or college.