How can I scan a thing and modify it or scan two things and compare them?
While I'm not sure what you mean by I want to scan a thing and modify it a little to see its effect. In general, a 3D scan typically results in a series of raw points. You'll need to convert the 3D point cloud into a Mesh before you can really do any solid modeling with it. Here are a few OpenSource programs that you could potentially use:


MeshLab
Blender
CloudCompare


To compare two scans, you could use the raw data in CloudCompare as well.

However, without knowing what your intended use is, it's difficult to tell how to help you. Please consider updating your response with a bit more of your own research so that we may definitively answer your question.