Right-click on the .001 file and select "Extract" or "Open archive." The software will automatically "stitch" the .002 part back together to reveal the original folder or file.

Download and install a tool like 7-Zip (Windows) or The Unarchiver (Mac).

If you only have the .002 file, you will receive an "Unexpected end of data" or "Cannot open file as archive" error because the "map" of the contents is stored in the .001 file.