Edit text of WordPress “Register” button

To change a particular string, you can use the gettext hook:

/*
 * Using priority 20 to override the default 10.
 * If another theme or plugin has a higher number,
 * you might need to raise this.
 */
add_filter( 'gettext', 'wpse_change_strings', 20 );

function wpse_change_strings( $translated_text ) {
    // If this is the specific string to change, update it.
    if ( 'Register' === $translated_text ) {
        // Change this to your desired string.
        $translated_text="Updated Call to Action";
    }

    // Always return the text, whether we changed it or not.
    return $translated_text;
}

The above is case-sensitive. You could use strtolower($translated_text) to compare if you’d like to change the text regardless of capitalization.