How to modify a mesh in Fusion 360.

This is an alternative to how to modify a mesh file when the shape is too complex. If any step is unclear, please leave your questions in the comments, and I'll try to respond ASAP. :)
These are the things I uploaded content for free:
makerworld.com/en/@jhdezrendon
www.printables.com/@JessR_189944
grabcad.com/jess.rendon-1
And here I uploaded some cool stuff:
cults3d.com/en/users/jhdezrendon/3d-models