You can manage sources and their order using the Dolby Dimension's Mobile App 'Manage Sources' page.  This will display a list of source devices that have been paired to Dolby Dimension.  You can reorder the list of devices by clicking the 'Edit' link in the right top corner of the mobile screen.  Paired sources can also be removed/deleted.  When finished editing your Source Device pairings, you may be prompted to reconnect by pressing the desired Source Button again.

To manually change source assignments, press and hold one of the three Source Buttons to enter Pairing Mode.  Once in Pairing Mode, you can pair a new Source Device to that Source Button.  This method will remove the prior Source Device from the Source Button selected.