I seem to recall that a rollback is sometimes possible from the hidden diagnostic menu, for a brief period after an upgrade. I haven't managed to do this the one time I tried.
Otherwise I think Roku support can set up a rollback remotely for specific device serial number, and had to do that early in 10.5 rollout when problems emerged, or accelerated a hotfix for affected users which was then rolled up in minor 10.5 update later.
Not a Roku employee