They should be phrases so you can go to Admin CP > Appearance > Search Phrases
Search for the text that's already there e.g. "Showcase"
Quite a few might come up, but to give you a clue, the phrase name for that is "nflj_showcase_menu_main_tab".
Change that to whatever you like. That should update the Showcase tab. Sometimes it might be trial and error, or, of course, you could just ask the Developer themselves how you would change it.
bobster who created Showcase, for example, provides excellent support very quickly.