To reverse direction and retrace one's steps.
Realising he had forgotten his briefcase, he turned back and re-entered the office.
To cause to reverse direction and retrace one's steps.
The barrage of machine-gun fire turned back the encroaching soldiers.
To return to a previous state of being.
He stopped drinking for a couple of years, but now he has turned back to his old ways.
To prevent or refuse to allow passage or progress.
The soldiers turned back all the refugees at the frontier.
To adjust to a previous setting.
I love that song: turn back to it!
To fold something back; to fold down.
When you make the bed, please always turn the sheet back over the blanket.
To give back; to return.