Find a Home & Look Into the Downgrade Process
And finally, you'll want to find a home.
You'll want to find a forum where you can go for long-term feedback about your specific Marshmallow update. If you aren't part of a community, now would be a great time to get familiar with a forum like XDA-Developers or Android Central Forums.
We also recommend getting familiar with the forums that U.S. carriers provide. There's a lot of noise there but there are also some very helpful people/customer service agents that will be able to assist you and answer querstions you might have.
Forums are a great place to find fixes and feedback and they're also a great place to find additional help if you run into problems with Marshmallow. Track them down right now, bookmark them, and return when and if you need assistance.