Can I backup my OS from Marshmallow back to Lollipop?

[AC Question]

I would like Lollipop back

 

[cybersa]

If you want to backup your current os,You need Recovery like CMW,TWRP.
After backup is done,you can flash Stock Rom to get back to Lollipop.

You need to search for Stock Rom based on your mobile model.

 

[Rukbat]

You can flash the older ROM on many phones without changing the recovery.

The problem is that an update isn't an addition to the older OS that you can remove, it's an entirely new OS. There's nothing to "back up" to.