As you work with a kit, you may want to separate some of our models into smaller pieces for more advanced editing. Whether you are trying to assign materials to a smaller piece of a model, or edit the geometry itself, all 3D software have different tools to accomplish this.
For each 3D Software, look for a tool called "Separate Mesh." The button is in different places for different software, so it's easiest to check your 3D software's documentation to find the proper ways of doing this. You can also reach out to our community on Discord if you need help.