Backup Battery

WARNING: This page is about a different car, the 2017 Subaru Outback and 2017 Subaru Legacy. However, it is still accessible from the selected car via links, so may be relevant.

Disconnect the ground terminal from battery sensor. Ref. to BATTERY
Remove the data communication module. Ref. to DATA COMMUNICATION MODULE , REMOVAL, Data Communication Module.
Remove the backup battery.
1. Remove the screws, and then remove the battery cover.
  Preparation tool:
  
  TORX® T10
2. Disconnect the connector and remove the backup battery.
Fig 1: Identifying Backup Battery Cover Screws And Connector
Courtesy of SUBARU OF AMERICA, INC.