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 tym wen i do revsision for same wbscode(pcode) i need to update the revisionnumber as 2..SO ON..i am doing it using javascript.plz help me