If you wish to have a different label for the 'Marketplace' tab, you can do so by using a Theme card.
Simply edit your theme card (or add a new one) by going to the Themes section on the left column.
In the form for the theme card, there is a section “Do you want a different lebel for the 'Marketplace' tab?" where you can input the preferred name for Marketplace tab, and any translations that my be necessary if you're using multiple languages in your guidebook(s).
Remember to save the theme, and make sure it's assigned to the guidebook(s) and that's it!
Hope this helps, and please let us know if you have any question via pop up chat on the bottom right of the host interface, or via email to email@example.com.