Basically you'd use two variables representingthe current maximum and minimum, initialize them appropriately and compare each vectort element to them, e.g.
#include <limits.h>
int min = INT_MAX;
int max = INT_MIN;
// ...
Now, if your current user_num[i] is larger than the current 'max', replace 'max' with 'user_num[i]' and use the same method for 'min'.