Q:

replace heading "Register" to "Signup"

i would like to replace heading "Register" to "Signup"  

A:

You can use the following code to change the text. It uses the get_textfilter to change all available translatable strings. Tested and it Works.
function custom_wc_translations( $translated_text, $text, $domain ) {
    switch ($translated_text) {
      case 'Register' :
        $translated_text = __('Sign Up', 'Domain');
        break;
    }
  return $translated_text;
}
add_filter( 'gettext', 'custom_wc_translations', 20, 3 );

