How do i change the channel of an 1/10 scale Exceed RC Forza buggy?

I bought 2 exceed rc buggy's named above and if i try to race together i know i am going to destroy the cars because i am going to end up controlling the other car and the other person my car. So i wanted to know how if you can to change the channels on my receiver and transmitter (i have a 2.4 GHz receiver/transmitter)by the way they are nitro gas powered

You do not have to do anything to "change the channel". The 2.4 GHz receivers/transmitters use a technology called spread spectrum and the receivers/transmitters are binded to one another with a unique code. The chances of you jumping on another code is rare since there are about 134 million possible codes.