I have a site collection
http://SPL-GT/sites/CP and would like to change it to a shorter version of
http://SPL-Sharepoint. How this can be done?
You can do it with Powershell.
This Blog might help you. https://www.sharepointrelated.com/2015/03/30/change-site-collection-url-sharepoint/
it should not have managed path otherwise you need to follow the backup and restore approach.