Its Ghille suit not guilly suit.
I'm assuming that you mean how to change your ghillie suit in Modern Warfare 2. The only way you can change the ghillie suit is by completing the GHILLIE IN THE MIST challenge. Doing so will change your ghille suit. NOTE: That you have to complete about Ghillie in the Mist I-II-III. To achieve ghillie in the mist, you have to get the aquirred number of "One Shot- One Kill". It just changes the ghillie suit so that it has leaves on it. Not very good on urban maps because you will be like a walking bush to the enemy
Tip: Use the Barret .50 cal with Thermal and FMJ and you will be getting the new ghillie suit in no time.