Back up your images files, they are NOT Stored In LR they are stored someplace on your hard drive, and then make a copy of the LR Catalog file.
Do all those backups to an external drive. Also take note of the actual PATH to the image files. If you have not manually selected a storage location for your image file then by default LR stores them in your UserName folder Under the Users folder on the ROOT of the system drive in a folder named Pictures\Lightroom (Backslash being a forward slash on a Mac).
For everything to work easily once you reinstall LR you must return both the image files and the catalog file to the Same PATH they came from.