editing stl files

Hello everyone, i am new to the 3d printing, i have the Lulzbot A0-101and downloaded the software Slic3r and Pronterface.
I have made a few prints and they are coming out ok so far, but could use a few tweaks, i have downloaded some files from thinkiverse and i would like to know if theres a easy way to just make a simple modification for example an iphone case, i would like to be able to edit the back of the case to a cut out looking image or text and then be able to print it out, which software do i need? or can i do it with the 2 i have?
thank you very much, all information appretiated!!

Lots of people use Bender, mainly because its free. Some thingiverse files will include source code, so might need a different one to edit those.

do you mean the skp files? or the stp files, that I can edit in blender? thank you

Well depending on if source is included they should be in STL format. If the source is included it could be from 3d Studio Max, Cinema 4d, Rhino 3d, Blender, Autodesk Inventor, Lightwave, free cad, etc. Blender and most 3d programs, except for Inventor and 123d Design will open up and allow editing of STL files, but they will be polygon edits. The original format is the best, but lots are not included.

any idea how to open an stl file in blender? i seem to cant find the option, thank you

It’s in File --> Import.