I think it’s only available when there is an upgrade firmware available. If your fw is already on the latest update them it just says"your firmware is up to date!" and shows the app version. Then you can’t click on the have a problem button to get to the firmware versions to downgrade to. This is what I’ve seen on the non beta app.
I think the revert to earlier firmware should be an option when you have the latest firmware… Not just when you have an older firmware.