I successfully rolled back from experimental V5 to V 4.20.
I used the same method as Jamie but under firmware checking you have to hold the ALT button.
There you will have the option to roll back to latest stable version.
Remember to hit “next” button once you have checked the box…I hit the box itself resulting in “you already have the latest firmware”. Took me many attempts to find out that I hit the wrong button 
PS!! If you dont have a Homey Pro + subscription you will loose all your data doing this!
I have a subscription and was able to restore all my settings from latest V 4.20 backup after rollback.
PS2!! Make sure you have disabled to experimental tab under homey developer - if not homey will update to V5 again 
