I'll show you two ways to reverse a vector. The built-in way is reverse(v.begin(), v.end()) from the algorithm library which flips the vector in-place. This runs in linear time and modifies the original.
To reverse without changing the original, create a new vector with vector rev(v.rbegin(), v.rend()) using reverse iterators. You can also manually swap elements from both ends with two pointers moving toward the middle.
Reverse the first half by swapping v[i] with v[n-1-i] until they meet. This is slower to write but teaches how reverse works internally.