For this workshop, you'll want to download your "mesh package" from 123D Catch. It should contain your 2D image files as well as your model in STL and OBJ format. You'll be editing the OBJ file.
Next, download and install Meshmixer, and then open your OBJ file in the application. Together with the instructions I provided during class, use the manual, hotkey reference, selection tool reference, and online tutorials/videos to begin editing your 3D model.
As you proceed with the editing process, be sure to regularly save your file. Also, consider taking snapshots (or screengrabs) of your work and decisions as you progress. These image files will be useful as you proceed with your DHUM 250 research log.
Of note, I've found this video (especially 2:56 forward), by Tinkernut, helpful when editing models in Meshmixer: