The app selector is located on the record editor within the Fulcrum web app. The label for the app selector will be the name of the app that you are currently viewing the data for.
Changing what data you are viewing
To change which app you are viewing data for you can click on the app selector to view a pick list of the apps and repeatable sections within those apps. Clicking the name of another app will load the data/records contained within app into the record editor. Clicking on the name of a repeatable section will load all the data contained within that repeatable section within the record editor.
Within the pick list, apps have bold text and icons, if set. The text for repeatable sections is normal.