I had some problems while downgrading so here are my steps.

Documentation is not really clear about this, so here are the steps.

1. from fxos, reformat system
   firepower-2110# connect local-mgmt
   firepower-2110(local-mgmt)# format everything
   All configuration and bootable images will be lost.
   Do you still want to format? (yes/no):yes
2. enter rommon and boot via tftp
   rommon 1 > address 10.86.118.4
   rommon 2 > netmask 255.255.250.0
   rommon 3 > server 10.86.118.21
   rommon 4 > gateway 10.86.118.1
   rommon 5 > file cisco-asa-fp2k.9.8.2.SPA
   rommon 6 > set
   rommon 7 > sync
   rommon 8 > tftp -b
3. everything is erased so you have to reconfigure your mgmt interface
   firepower-2110# scope fabric-interconnect a
   firepower /fabric-interconnect* # set out-of-band static ip 10.86.118.4 netmask 255.255.250.0 gw 10.86.118.21
   firepower /fabric-interconnect* # commit-buffer
4. download image (could not get USB to mount so I used ftp)
   firepower # scope firmware
   firepower /firmware # download image ftp://user@10.86.118.21/cisco-asa-fp2k.9.8.2.SPA
   check status:
   firepower /firmware # show download-task
5. install image
   look at version you want to install:
   firepower /firmware # show package
   firepower /firmware # scope auto-install
   firepower /firmware/auto-install # install security-pack version
   check status (wait for Update Software Pack Completed):
   firepower /firmware # show
6. configure ftd/initial configuration
   firepower /firmware # connect ftd