I have used vTiger WebForms module to insert new leads inside the vTiger system, now I need to perform an update for these leads - is there any to do this operation?
I did not find any reference for this operation...
You must download and read documentation to make connection via web services with vtiger crm to perform update/delete/insert download from here:
Look at Web services documentation.
May be you have to change index file in webforms folder and call appropriate api's over there. Before that take up of your files and i think you need to register an that operation before you do any CRUD operations.