How to revert firmware from 24.19 to 21.18?

Numerous grave bugs in 24.18 and 24.19 make Edge 840 unusable: Wi-Fi requires removal and readding after every start, or stales in "Syncing". No activities or courses synced unless you do this manually. Hangs during rides requiring hard restart.

I would like to revert to 21.18 which actually worked. If not possible, considering requesting money return on warranty, as Garmin broke my device to effectively unusable. How to arrange that?