If you are looking to lower your firmware to gain jailbreak compatibility, the community-accepted method requires and specialized hardware.
Public jailbreaks exist for specific firmwares (most notably firmware 9.00 and certain entry points for firmwares up to 11.00). ps4 tool downgrade v100 exe link download
Downgrading the actual system firmware (e.g., from 12.0 to 9.0) is significantly more difficult and generally cannot be done with a simple Hardware Requirements If you are looking to lower your firmware