Is there a way to revert an android version update without flashing a ROM?
There's no sort of "rollback" feature on Android. Due to device space limitations the old OS image is not kept around.

So to downgrade you'd have to flash. It's not necessarily the case that you'll lose your data when doing this, but it depends what the ROM's updater script does and whether the ROM will work with the data left by the previous version (you might need to manually factory reset, which is generally recommended before flashing). As Andrew noted in the comments, unlocking your bootloader will wipe the device so if you need to do that to flash then you're out of luck. As such it is always recommended that you do a backup before flashing.