Changing University

Hi guys,
Need some advice, I have a Visa interview in the coming days. I already have secured admission in masters in a US university. Now, I have just received news of anther admit offered to me in a better institute. What should I do to move ahead with the process of changing the university?

Sorry cannot help with you this. And as I can see that you posted this query a while back. You must have done with the process. Can you tell us how to proceed with this. I have few juniors from my college planning to study in the US.
It will be a great help.
Thanks in advance.