I'm not with Laminar Research, just a fellow simmer in the UK :-)
The whole X-Plane 11 root directory folder is completely standalone and does not write any files to your PC's registry or install any programs in the normal sense.
So, if you wish to remove the entire X-Plane 11 root directory from your local C:Drive, just go into File Explorer (or whatever it is called in MAC: File Manager??), click on the X-Plane 11 folder and delete it. This may take a while due to the number of files etc, but everything associated with X-Plane 11 should be gone afterwards.
TAKE CARE here and only do this if you're absolutely certain that this is what you want to do? If you're not sure, then save a copy of the whole X-Plane 11 root directory folder somewhere safe on another drive, just in case you need to refer to it again.
For example, you may have certain addon aircraft types that you wish to use in the fresh copy of X-Plane11, or you may have addon scenery etc.?
Anyway, once everything has been deleted, simply download a new digital copy of X-Plane 11 from LR's website and use your original product key to activate it.
If you were using the DVD set previously, just re-install X-Plane 11 again as you did before.
If you purchased your copy of X-Plane 11 from another 3rd party source, speak to them first before doing anything?
Hope this helps and if you have any questions, please add to this post?