You have to downgrade all firmwares (sensors hub, wifi, gps, ant/ble/bt) not just the main software. If you want to downgrade to 3.00, you need to know all the other firmware versions which works with that main software.
When you have all the required versions, copy the main software to the Garmin folder as GUPDATE.GCD and copy all of the other firmwares to the RemoteSW folder. After disconnecting, it should start the downgrade process.
Note that, you won't be able to use any CIQ apps after the downgrade, unless you have an Apps folder backup from which you can copy back the previous CIQ apps you used before the upgrade.
Unfortunately I don't know that as I have a Marq not a Fenix6, but hopefully you will get responses from the Fenix6 users.
Which model do you have?
Alternatively, you can still contact with Garmin Support, they should be able to confirm the version numbers.