How do i edit the location of my Catalog?

The easiest way to do it would be to copy the folder that contains your catalog to the external hard drive. Of course, you would do this with Lightroom being closed. Then, open the folder on the external hard drive and double-click on the catalog file. This will open the catalog file from its new location. From that point onward Lightroom will call the catalog from the new location. But that isn't going to move the image files to the external hard drive. That will only move the catalog and associated catalog files. After you have verified that everything is running fine, you can delete that folder from its original location.

If you have your images organized within one master folder, you can COPY that master folder over to the external hard drive. Then using Lightroom, you can right-click on that master folder and choose the option to update the folder location. All you have to do is point Lightroom to the folder on the external hard drive, and that will update the location for all of the subfolders as well. Then, after you have verified that everything is functioning properly you can remove the image folders from the internal hard drive as well. If the image folders are not organized within one master folder then you will have to update the folder location for each individual folder.