The Unenroll from Another Journey flow node allows you to unenroll a contact from another Journey. See Fig 1 below.

Fig 1

To select which secondary Journey the contact will be unenrolled from, click the Unenroll from Another Journey flow node once you have added it to the Journey work area. Doing so will bring up this popup ("Properties") on the right-hand side of the screen. See Fig 2 below.

Fig 2

Clicking into the "Journey" input field below the "Journey" label will allow you to select another Journey from which to unenroll the contact. Please note: Only ACTIVE journeys targeting the same database will be available as options.  See Fig 3 below.

Fig 3

Once you have finished with the action's setup, click the "Save Properties" button.