How To Go About Transferring Between Colleges

Hi

I’m an admitted student into MCS at Carnegie Mellon. At the time of applications, I thought I wanted to major in Physics, but then the current class I was taking got past mechanics and went into General Relativity, and then Quantum Mechanics (I’m taking a 300 level physics class at UW-Madison).
I started losing interest in physics and gained interest in Mechanical Engineering. I’m part of Rocket Club at my school so I’ve gotten very familiar with CAD programs and building airframes. So I would really like to transfer over to Mechanical Engineering, but when I applied I only applied to MCS.

How should I go about things, like choosing classes, if I want to make the switch as soon as possible?

Thanks!