You want to change the default version of SGOS on the ProxySG from the CLI.
The Management Console is inaccessible to change versions of SGOS and you will need to change the default version of SGOS on the ProxySG from the CLI. 
 

If Management Console is inaccessible and you want to change the version of SGOS running and restart the SG, connect to the serial port on the proxy (or SSH if SSH is functioning) and do the following commands:

ProxySG>enable
Enable Password:
ProxySG#config t
Enter configuration commands, one per line.  End with CTRL-Z.
ProxySG#(config)installed-systems
ProxySG#(config installed-systems)view
ProxySG Appliance Systems
1. Version: SGOS 6.1.3.1, Release ID: 56222
   Tuesday March 8 2011 00:53:52 UTC,
   Attributes: Signed, FIPS capable
   Boot Status: Last boot succeeded, Last Successful Boot: Wednesday April 13 2011 12:37:37 UTC
2. Version: SGOS 6.1.2.1, Release ID: 54104
   Tuesday January 18 2011 09:08:39 UTC,
   Attributes: Locked, Signed, FIPS capable
   Boot Status: Last boot succeeded, Last Successful Boot: Thursday January 27 2011 21:10:07 UTC
3. Version: SGOS 6.1.1.5, Release ID: 52444
   Friday December 3 2010 23:18:50 UTC,
   Attributes: Locked, Signed
   Boot Status: Last boot succeeded, Last Successful Boot: Thursday December 23 2010 17:31:37 UTC
4. Version: SGOS 5.4.4.7, Release ID: 49360
   Thursday August 19 2010 04:27:40 UTC,
   Attributes: Locked, FIPS capable
   Boot Status: Last boot succeeded, Last Successful Boot: Sunday September 19 2010 04:41:47 UTC
5. Version: SGOS 5.4.4.1, Release ID: 45872
   Wednesday May 26 2010 07:52:07 UTC,
   Attributes: Locked, FIPS capable
   Boot Status: Last boot succeeded, Last Successful Boot: Tuesday August 31 2010 21:58:51 UTC
Default system to run on next hardware restart: 1
Default replacement being used. (oldest unlocked system)
Current running system: 1
Enforce signed: Disabled
ProxySG#(config installed-systems)default 2
  ok
ProxySG#(config installed-systems)exit
ProxySG#(config)exit
ProxySG#restart regular   <<---  NOTE:  This command will cause the proxy to restart.