What could be happen if we can change the return value 0 to 1?

Answer 1

If you are talking about main() then:-

On some IDEs it doesn't matter but on some it does
Because now a days compiler use the return value of main as exit code. Return 0 means code is compiled without any error, any other value means there is an error and the error is identified using return value

Answer 2

0 indicates 'Successful'.
1 indicates 'Error'

Answer 3

Well, it is correct that 1 indicates error.
But seriously if you try it out practically, nothing happens. The ouput remains quite the same.