Changing Parent Slug for Galleries
Changing Parent Slugs is a very simple process. First, go to Flotheme – Gallery – Miscellaneous tab. There you will find 3 fields.
Replace the word gallery with portfolio or any other for that matter, and click Update. This will then affect the website URL Slugs. see screenshots below
Note that in some cases these changes do not apply until you re-save the permalink structure.
To do that, just head over to Settings > Permalinks and click Save Changes without changing anything: