How do I turn my phone back to the stock ROM?

Hey,
I'm a total noob on this so here's my problem
I have a oneplusone
It had CM 11 stock
Now I updated it to cm12, all bugs, to cm12.1 same bugs.
How do I turn it back to CM11
It's not rooted
I tried several but it doesn't work for my,
Can some one help me in the right direction?
Tnx

Sent from my A0001 using Tapatalk