What kind of changes? I mean, you can go into the precomp and do stuff in there or you can do stuff in the main comp.
You can't "undo" precomposed clips (other than the undo command [that is, Edit>Undo or ctrl+z on PC cmd+z on Mac]), but you can copy them from the precomp and paste them back into the main comp.
If you're new to AE, you should really get a firm grasp on the basics before you do anything else. Lynda.com and Digital Tutors (now called Plural Sight, I think) have some great courses. If you don't want to pay anything Adobe has some free getting started lessons on Adobe.com, but I quite like the series of tutorials Andrew Devis did on CreativeCOW.net (you can find them here) They were done a while ago, but the basic functionality of AE hasn't changed much since then.
I tried going back into the precomp to make changes but no video displays. I also tried copy video from precomp to main but nothing happens. I know it shouldnt be this hard and im some how missing the steps. thanks for the link. Ill check it out!
Keep in mind that After Effects is the most complex piece of software Adobe makes. It's not the kind of thing you can pick up in a day and it's not the type of thing you can pick up just by messing around - like you can with Photoshop. So, be prepared to put in quite a bit of time before you can really get into using it.
Yeh Im starting to realize that. Ive been good so far with following tutorials but you are right. Understanding the fundamentals is crucial. This is my second week in actually learning and using this powerful app! I love it but yeh its complex for sure.