changing university course - biomed to computer science

i was supposed to be starting university this year and was doing biomedical sciences. i had confirmed places at St Georges and an unconditional offer at Birkbeck and had rejected Brunel, Westminster and Queen Mary's. However, after thinking the whole course over during lockdown, i feel like i might have made a mistake and kinda want to do computer science instead, and after looking at the league tables for the course for which uni is the best one for it in ldn and do clearing, brunel and westminster (who i rejected :/) seem to have come out on top. Does anyone have any advice on how to word emails to these two universities about this as well as any advice on what to say to the two unis i had previously confirmed? Would I have to email UCAS too?

Are you getting results this year, or do you have your results already and had unconditional offers from all three unis?

Clearing doesn't open until 6th July, so what you would be looking at right now is a course change. You'd need to call the unis you're interested in (don't worry about having previously rejected them, they won't take it personally) and see if they will make you an offer for CompSci. If they are willing to do so, then you'll need to contact UCAS to process the swap, and you'll need permission from the unis where you currently hold offers.

If you want to use clearing, the process is a bit different. Once you have your place at St Georges or Birkbeck confirmed, you can self release into clearing. You'd then need to phone the unis you're interested in, and see if they will make an offer for CompSci. You can then add any offers you get via UCAS- however, you'd lose your origional offers, so could end up with nothing.

i should be getting results in August hopefully. If i did call the universities, what points would i have to mention? also after calling them, would i have to call st georges and birkbeck to let them know of the change?