Open the STL file in a 3D editing program
Import the STL into the software
Use mesh repair tools to fix holes, non-manifold edges, or flipped normals
Use sculpting tools to reshape the model
Use transform tools to move, rotate, scale, or mirror the model
Use cutting or slicing tools to remove unwanted parts
Use boolean tools to combine or subtract shapes
Use smoothing or remeshing tools to improve the mesh
Export the edited model as a new STL file
Check the edited STL in a slicer or mesh viewer before printing