It's really a very simple process. You open your catalog(s) using Lightroom 6. The program will prompt you to create an updated copy of your catalog. You should allow that to happen. Lightroom 6 will then create a new copy of the catalog and will append a "-2" to the end of the filename. Then new catalog will be opened and LR6, and will contain all of the work that you have done. The old catalog will remain unchanged and usable by Lightroom 5.