Did you enrol already? There should be an admissions email somewhere for the university who you can tell you want to withdraw asap.
Also, have you got clearing options? It may be important to look at that BEFORE you choose to permanently withdraw or email anyone. You can call other unis clearing numbers and see if they can offer you a place before you choose to update your UCAS/ask you current uni to release you into clearing.
If you have student finance, try calling/emailing them as well beforehand to understand what you would need to do.