I need some help changing a .stl model

It's not so much a specific feature, but a combination of them. In this case, after reducing the triangle count (IMPORTANT!), I selected W for wireframe view, then used the select tool to select all the triangles surrounding the depressed lettering. After consulting with a mental health professional, it was decided the best approach was to delete the selected triangles. This removed the support structure for the depressed triangles, causing them to fail the Analysis/Inspector. Automatic repair removed the letters and closed over the resultant gap. It's important to avoid surrounding more than one letter at a time, as it will result in a repair that creates a depressed mostly flat area, not longer planar. There may be some remnants of my failure in that aspect, although I performed the delete and repair over those valleys with great success.