Heya, I've just finished my Re-Sat year 12
about to go in to year 13 now. If you want to re sit the year you'll need to go into your school and talk to them about it. Schools don't really like it as its not an easy thing to do and out of the 7 of us in my year that dropped this year, only 3 are still at school this coming September. If you are really committed to it, i would recommend it. I did the same as you and got BCEE in my first year 12. I carried on the B and changed my options. this year i got B in the A2 biology and ABD in the others which means my grade predictions for Uni (you only take the best 3 for UCAS offers) will be A*AB which is so much better than my original options :P )
I've also changed Degree options. If you are thinking of re sitting be prepared to find new friends in your new year early, It makes it slightly less scary when your "old" friends leave school after exams. Its not an easy option but it is doable if you really want to do well and are able to really fight for it. Let me know if you need any more info
Good Luck!