Hi, so im new to using forge for modding, but when i used mcp i was able to change the original minecraft folders. Basically i want to change the GuiButton, but im not able to edit any of the original minecraft files. On mcp i could just click on GuiButton.java and do what i wanteed with it, but now its GuiButton.class and im not able to edit it. Is there any way of editing this file, and any of the other mineraft files or no? If not is there any other way i can try to have custom gui buttons and what not.

You don't. That's the point. Minecraft should be treated as a library. You should write your mod in a way that doesn't break other things.

If you can't do something then think about what you're doing and see if it'd be a good fit to include in Forge. 

As Forge is designed to fill in the gaps where vanilla api fails.

 

Also, you don't need to edit GuiButton to have a "custom gui button" This is java.. you can extend the object and change whatever you want. To use those changes just use your subclass instead of vanilla's.