Rejecting firm offer, applying through clearing

I know results day is quite far away, however can someone walk me through the process of rejecting my firm/insurance choice then applying through clearing? Would I need to reject my choices BEFORE I apply do clearing and if this is the case would there be a chance I could miss the spot in my preferred uni at clearing? I've had a change in heart and wanted to apply elsewhere however the date has already past and want to look into how the clearing process works incase I make a mistake on results day

I know results day is quite far away, however can someone walk me through the process of rejecting my firm/insurance choice then applying through clearing? Would I need to reject my choices BEFORE I apply do clearing and if this is the case would there be a chance I could miss the spot in my preferred uni at clearing? I've had a change in heart and wanted to apply elsewhere however the date has already past and want to look into how the clearing process works incase I make a mistake on results day

Is the course you’re interested in available in Extra?

How am I able to see whether it is? Sorry I can't seem to find it

Search for courses on UCAS then adjust the results filter to only show courses with vacancies

Yep found it and yes it is, sorry for the confusion.

In that case ring them up tomorrow and explain the situation and ask if they are accepting applications through Extra (sometimes universities forget to remove courses when they’re full) and see what they say about applying.
No need to wait until clearing (and there’s no point in hanging onto offers for a course you don’t want to do anymore).
Worst case scenario is they don’t make you an offer so you can apply elsewhere in extra or wait and try again in clearing and if that doesn’t work then take a year out to reapply. Extra gives you an opportunity to have your place sorted out before clearing (and doesn’t stop you using clearing too!)

Thank you for your help, this is really helpful. I'll give them a call later this week.
Another question (thank you for your patience with me) but as the course is available through extra, could it then be likely that the course would therefore be available at clearing. Assuming that they haven't forgot to remove it, and I understand that it is still less likely to get an offer if I wait till then. This is a hypothetical question but I'd just like your opinion on it.
Thanks again for the help and sorry for all the hassle.