You missed two things. First - in order to swap two array items, you need to store item somewhere. Otherwise you just replace one item with other. Second - arrays have indexes from 0
to Length - 1
. So you need to subtract 1 from second item index:
for (int i = 0; i < numbers.Length / 2; i++)
{
int x = numbers[i];
numbers[i] = numbers[numbers.Length - i - 1];
numbers[numbers.Length - i - 1] = x;
}