In order to switch from University marketplace to Open marketplace or vice versa, just head to 'Settings'. There you have the option of moving between marketplaces.
If you want to stick with University but see every items from the Open marketplace too, you can find this on the same section.
Likewise, if you are moving to a different uni and just want to see items at your new campus, guess where you need to look? That's right, the 'Settings' page!
Also, if you want to log out of the app and log back in, the App will automatically take you to the welcome screen. From here you will be able to choose your new marketplace.