Yes but no.
You can usually change even within a few weeks into sixth form, HOWEVER, it depends if there is space for you to switch (which usually you will be able to)
On results day, you go and enroll at the sixth form/ college either in person or online(via a forms) what you need to do is basically say I want to do these subjects and they probably will accept you.
On result day I enrolled at 2 places so you can enroll elsewhere and then go to the your preferred one/ the one that offers you what you want, and tell the other one your not going.
Basically they probably will - but be prepared for no, wherthere you have different subject in mind or a different college
Hope this helps - what you need to do now is just focus on your exams and deal with this later.