You can't. Factory reset is the only way.
Heh, well, it may have clearly said it, but there ain't no prayer I can clearly remember.Quoting to agree... when you encrypted the phone, there was a message that clearly said this - that once you encrypt, the only way to decrypt is to factory reset the phone. See the screen images here: Encrypt your android 5.0 Lollipop device to increase security and secure data.
I remember that I encrypted right away upon getting the phone. Had to, really, if I wanted to access my company's email on it (required by the company). That job left me last week, but the encryption will have to stay. ;-)Trust me, I plowed through that screen without fully reading the first time I encrypted a phone, too.