How to turn a mesh into a low poly mesh I imported a lego build from LeoCAD into Blender and it has LOTS of unnecessary vertexes.

 

How can I delete all the vertexes inside the mesh, but keep all the vert's on the outside, so it looks normal from the outside but without all the vert's inside
Try the decimate modifier.

Select your mesh and go to the modifiers tab, where you can add the decimate modifier. Play around with the settings until the you are satisfied with the poly count, while retaining a good look.

Learn more about the decimate modifier here: https://docs.blender.org/manual/ja/dev/modeling/modifiers/generate/decimate.html