Pretty simple just go to this link: https://www.youtube.com/onechannel
Then scroll all the way to the bottom.
If you are on the old channel layout it will say:
Switch to the new YouTube channel design. Get Started.
If you are on the new one channel layout it will say:
You're already using the new channels design, but can switch back to the previous design if you'd like. Switch Back.
Just click the Switch Back button.
The one channel layout is now permanent and can not be changed.