How to edit wp-signup.php content using plugin

you can try these hooks:

// 
// remove label label[for=signupuser] and #signupuser on wp-signup.php with CSS
// 
add_action('signup_header','my_signup');
function my_signup() {
    echo "<style>label[for=signupuser],#signupuser{display:none !important;}</style>";
}

//
// edit text displayed on wp-signup.php 
//
add_filter( 'gettext', 'my_change_text', 20 ,3);
function my_change_text( $translated_text, $text, $domain ) {
    if ( $domain =="your_site_name") { // EDIT
        if( trim($translated_text)=="Get your own %s account in seconds"){
                $translated_text="Fields with an asterisk will be shown on your website for the world to see";
        }
    }
    return $translated_text;
}

where you will have to edit the value $domain ==”your_site_name”.

This will give you the following result (see the attached image):