I'll show you reverse(v.begin(), v.end()) to flip element order in place. The first element becomes last and the last becomes first. The middle elements swap so. You can reverse part of a container: reverse(v.begin() + 2, v.begin() + 5) flips only elements at indices 2, 3, and 4 while leaving other positions unchanged.
This runs in time since it touches each element once. You'll often use it after sorting to get descending order, though custom comparators work more directly.