Console access is required to revert to an earlier level as it requires interrupting the boot process and re-imaging. There is no inline downgrade process, much less one you can perform remotely.
Here is an older tech note on the re-image process:
http://www.cisco.com/c/en/us/support/docs/security/firesight-management-center/118308-technote-firesight-00.html
It's still accurate regarding the process even though it references older versions. You would have to restore using "Sourcefire_3D_Device_S3-5.4.0-763-Restore.iso" and then patch it to 5.4.0.9.
The iso can be found here:
https://software.cisco.com/download/release.html?mdfid=286261236&flowid=81829&softwareid=286271056&release=6.1.0&relind=AVAILABLE&rellifecycle=&reltype=latest
You are better off going to a more recent version than from an old one to an even older one. You can run the current 6.2 on the 7020 appliance.
