need to update the item in the list

I have a list called revision. It has a columns called wbscode and revisionnumber. Wbscode contains code of the project(eg.pcode1) and revisionnumber is to store revision number done for the pcode. Initially revisionnumber is to be set to 1 for pcode for 1st revision. Next time when I do revision for same wbscode(pcode) I need to update the revisionnumber as 2..SO ON.. I am doing it using javascript. Please help me