bash-4.1$ cat Rozuur.c
main(c){while(c=getchar(),~c)putchar(c-64u<27?c+32:c);}

bash-4.1$ cc Rozuur.c

bash-4.1$ ./a.out <<< 'Hello @ WORLD !'
hello ` world !

bash-4.1$ sed -i s/64/65/ Rozuur.c

bash-4.1$ cat Rozuur.c
main(c){while(c=getchar(),~c)putchar(c-65u<27?c+32:c);}

bash-4.1$ cc Rozuur.c

bash-4.1$ ./a.out <<< 'Hello @ WORLD !'
hello @ world !