It *is* possible to downgrade a UM3, @kevin-osborn, but it's difficult because the whole process changed in 5.2.  So if you are a version older than 5.2 it's easy to downgrade and if you are at version 6.0 (in the future some day) it should be easy to downgrade to 5.2 but no lower.   Anyway to downgrade across this boundary is more involved.   You need to remove the bottom cover and you need a micro-sd card and special software on your pc/mac/linux machine to create a boot pa