How to modify STL files

Hi all,

I am a CAD designer by trade. I'm pretty proficient at Creo/ProE that I use at work.
I have Fusion 360 on my personal computer and I'm pretty good at stumbling thru that too.

I can't find any way in either software to modify stl files.
I can open them - look at them - measure them, but I cannot select surfaces to build on or cut thru.
The software doesn't recognize them as a solid or even as surfaces or points to build on.

There has to be a way to do it or convert them to something that can be used, right?
What am I missing.

Thanks
Donna

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.

Thanks for the response. This has been driving me nuts since I started 3d printing. Sometimes it's just as simple as wanting to put a mounting hole thru something.

I have used the mesh in Fusion, but I couldn't find it yesterday. Update moved it I guess.

I tried one - I think it was MeshMixer - a while back on an old computer, but ended up just removing it because I couldn't figure out how to use it. It's pretty obvious to me that the people who write the instructions and help for these softwares don't actually use the software. LOL.

Maybe I'll try Blender. I'm not trying to do anything complicated - just add or remove a little material - shouldn't be this hard.