Query - Change in University after visa approval

i have an admit from X university in california for masters in computer science program for fall 2015, i pay the sevis fee and apply for visa and the visa is approved. so i have my visa approved and stamped with X university.

Now i get an admit from Y university in california only for masters in computer science program. Now I want to pursue my masters program from Y university. what is the process to change the university after getting the visa approved for one university?
Do we have to get the sevis details changed by the university or apply for visa again, or can we do this india or after going to US? Kindly explain the entire process.
Need Help here.