Hi raramm, I was wondering this too, and found the solution on this forum written by the theme developer. Except for the plugin in my theme was called “mauer-bild-portfolio”. This worked perfectly for me. See below:
The slug base of the Project custom post type is defined in the “Mauer Stills Portfolio” plugin.
To change “proj” to your custom string please do the following:
In your Admin Panel go to Plugins -> Editor. Select “Mauer Stills Portfolio Plugin” in the dropdown (see screenshot).
Then find the line that’s pointed to in the screenshot (line 53). Type your own word instead of “proj” and click the “Update File” button.
Finally, in your Admin Panel go to Settings -> Permalinks and click the “Save Changes” button.
I’m glad you like the theme. I would be very grateful if you could rate it on ThemeForest — here.
This literally takes a moment, however, makes a big difference.