Software for .stl files?

I have some .stl files from another project that I want to edit into something for myself. What software should I use?
The images from the project are here: BlueLive Controller by timothyreese - Thingiverse
I've never done any 3D printing, but it's just simple changes so I think I'll get the hang of it quick.

After more research, I've started using blender.