Far too often, I've found it difficult to edit someone else's STL creations, unless some form of source files are included. Fusion 360 has a mesh tab which opens a world of tools for manipulating the imported model. I'm in the same boat with stumbling through Fusion 360 though and am not very proficient in so doing. Converting the mesh makes things a bit easier, but doesn't make it easy. Another aspect of Fusion 360 is that the software auto-updates and includes feature changes. Some of the feature changes also change the access location of the features, rendering internet help too frequently useless. On the free software side of things, Meshmixer and Blender will import STL files. The former is somewhat limited to what can be done, while the latter is nearly unlimited. The "unlimited" aspect can be applied to the learning curve for Blender, however. There are quite a few non-free editors such as Rhino3D, ZBrush and others. I'm overlooking a number of free and non-free editors due to memory failure at the moment.