시간을 되돌리면 => if you turn back time (a dependent clause, not a sentence)
- 시간을 = 시간(time) + 을(object-marking particle)
- 되돌리면 = 되돌리다(v, turn back; turn around) + -면(conditional verb ending, like "if (you) do (something)").
The phrase is not very common though, because -면 by itself is a simple conditional used for routine things.
For impossible things like turning back time, there are subjunctive-like expressions which better expresses it.
사간을 되돌린다면 무엇을 다시 해 보고 싶어? = if you were to turn back time, what would you try over again? (note 다면)
시간을 되롤릴 수 있다면 얼마나 좋을까. = How nice it would be if it was possible to turn back time.
Compare it with real life conditional:
- 내일 비가 오면 어떡하지? What do we do if it rains tomorrow?
- 네가 안 가면 나도 안 가. If you're not going, I won't go either.