modify the query string value in url

Hello,
I have url like this (test.aspx?id=1,2,3&st=3,4)
and I wanna add parameters to (id) query string and return same url (test.aspx?id=1,2,3,4,5&st=3,4)
and FYKI maybe I have more than two queries (so just get the url then update the query string after that return same url with same queries order)

Regards.