Urgent STL File Modification for 3D Printing (Change Outer Shape from Triangular to Circular)

I have an STL file that currently ends in a triangular outline. All I need is for that outer contour to become smoothly circular while the interior—especially the existing holes or cut-outs—stays exactly as it is... (Budget: €8 - €30 EUR, Jobs: 3D Animation, 3D CAD, 3D Design, 3D File Scaling, Slicing & Print Preparation, 3D Modelling, 3D Printing, 3D Rendering, 3ds Max, Blender, Fusion 360)