Revise Existing 3D Print Model

I already have the design in STL format and need it tweaked so it prints perfectly. Two specific updates are required: first, adjust certain dimensions to refine the overall shape and size; second, add or remove a few features that will make the model function the way I need once it comes off the printer.

Please work directly on the supplied STL—or import it into Blender, Fusion 360, SolidWorks, or any other tool you prefer—then return a clean, manifold file that slices without errors. An updated STL is the only mandatory deliverable, but if you can also share the editable source file that would be a welcome bonus.