isupper() character function is used to check if the argument contains uppercase letters (A-Z). This function returns zero if uppercase letters (A-Z) are not found.

    int isupper( int c );

   #include <stdio.h>
   #include <ctype.h>
   int main ()
     {
       char arr[]="ABCD.\n";
       char c;
       for (int i=0; arr[i]!='\0' ; )
        {
         if (isupper(c))
          {
            c='x';
            putchar (c);}
            i++;
          }
       return 0;
     }  

    xxxx.   

In the above example, it is used to check the uppercase letters (A-Z) and replace it with lowercase "x". The character check is made using the ctype facet.