how this custom toupper() function works?

I've seen following program that uses custom toupper() function.

#include <stdio.h> 
void my_toUpper(char* str, int index)
{
    *(str + index) &= ~32;
}
int main()
{
    char arr[] = "geeksquiz";
    my_toUpper(arr, 0);
    my_toUpper(arr, 5);
    printf("%s", arr);
    return 0;
}

How this function works exactly? I can't understand logic behind it. It will be good If someone explains it easily.