How to edit Minecraft's base files?

[MkDomain]

How to edit Minecraft's base files?

Please help with editing Minecraft's base files.

I was wondering if I could edit those files (like GuiMainMenu.java) throught ClientBase.
If there'se a way let me know it, if there's no way please make it :)

[superblaubeere27]

You have to use mixins tho. It might seem hard but it is easy once you understood it :D https://github.com/SpongePowered/Mixin/wiki/Introduction-to-Mixins---Understanding-Mixin-Architecture