Editing an STL in FreeCad

1. Click File->New
2. Click File->Import and select the STL you want to edit
3. Select Part workbench in tool bar
4. In Model tab, select your object (mesh), click Part->Create shape from mesh, check Sew shape amd give it the value 0.01
5. You no longer need the mesh, so right-click on it and click Delete
6. In Model tab, select your shape, click Part->Convert to solid
7. You no longer need the shape, so right-click on it and click Delete
8. In Model tab, select your solid, click Part->Create a copy->Refine shape
9. Now select Part designer workbench in tool bar
10. In Model tab, select your solid 001, click the tool Create a body

That's it, click Body in the model tab and start editing your solid.