How to reset phone back to runtime Dalvik

[AC Question]

A few days ago I switched my LG Realm LS620 runtime to ART it restarted (power off and then turned on) and a message that reads " Android is upgrading" shows up and under that it say optimizing app (number) of (number). When that's finished a lot of messages pop up saying google play backup stopped, android ui stopped, etc. So my question is how do i do a factory reset with phone turned off i already tried pressing Power+Volume Up.