Can I swop my university choice?

I sent the application to my 5 unis around 20 days ago, and now I realise that I want to swop one of my choices for a different university.

It still hasn't been past the 31 July deadline, so would it be possible for me to change university options?

no you can only swap within 14 days. Your other option would be to decline ALL your options and use extra when it opens to apply to 1 university. That is assuming the course you are interested is open then.