Resiiting a unit mentioned on ucas

So here's the deal, I applied and I've got all my offers so that's very very good and I am extrememly grateful however, I am now worried about my offers as the grades I have to get might be tough to get.
On my UCAS i mentioned my unit grades for chemistry and in one unit I got a B and on my ucas it says i have a B,can i resit this module to get an A even though it is mentioned on my ucas as a B grade not as pending.