A non-decreasing array requires that, for every i-th element in the array, it is true that v[i] < v[i+1]. We are only allowed to change at most 1 number in an array to turn it into a non-decreasing, therefore, at most one v[i] < v[i+1] can be false.
If in a given array only one v[i] < v[i+1] is false, then v[i-1] < v[i] is true, for i > 0. To be possible to turn the given array into a non-decreasing, we need to change v[i] to a number such that v[i-1] < v[i] < v[i+1] is true. Since the problem states the given array is an array of integers, it is enough to verify if v[i + 1] - v[i-1] > 1, i.e, if there exists a number between v[i-1] and v[i+1] that can be used to assign to v[i] such that v[i-1] < v[i] < v[i+1] is true, for some i > 0. If i = 0, we don't need to do the above check since we can pick any integer number lesser than v[i+1].
This solutions runs in O(n) for n being the length of the array, since we visit each array element only once.