How can I ungroup stl parts so that I can modify the design how I want

If STL combined from several parts You can split it. If STL is healed solid and consists only one shell - You can not modify it by splitting. But You may select its surfaces, make them solid and then modify - it is not easy but possible. The better way to ask Seller to help You in modifying...

Have a look into Mesh Mixer. It's free, it works with STL files, and you'll find plenty of videos on Youtube about how to edit STL files using it. If you aren't familiar with more pro-level software like Zbrush, Blender, or 3dCoat, then MeshMixer is a good solution.