Modifying a a 3D model STL file, making a part smaller

Hi, I'm new to 3D printing, got my Ender 3 v2 a few weeks ago. I'm also in the making my home smart. I'm all set up with Home Assistant, zigbee, z-wave and Shelly. The thing I've come across is that the Shelly Wall Switch 4 doesn't fit Schneider boxes for switches. I have the Schneider Exxact and Artic series, they are about the same size. The wall frame plate is called Schneider Exxact Combi frame. The inner part is taken out and the hole is then 70x70mm. I would like to edit the part so it's outer diameter is 70x70mm, but not just crop it, I want to save the "lip"I've downloaded this: https://www.thingiverse.com/thing:5022528 and also attached a photo. From there I would like to edit the second last 3D file so that the center hole stays the same but the outer diameter is 70x70mm, so it would fit my Schneider wall plate. I've installed blender, OpenSCAD and I actually had FreeCAD from before. I haven't done any design in CAD, just some light modification on electrical diagrams.EDIT: Just realized I posted in the wrong forum, should be in the Feedback / Assistance / Work-in-progress