save_user_data.php

<?php
add_action( 'personal_options_update', 'wpmu_usermeta_update' );
add_action( 'edit_user_profile_update', 'wpmu_usermeta_update' );

/**
 * Save user extra meta data
 *
 * @param $user_id The user ID of the user being edited
 *
 * @link https://developer.wordpress.org/reference/hooks/personal_options_update/
 * @link https://developer.wordpress.org/reference/hooks/edit_user_profile_update/
 */
function wpmu_usermeta_update( $user_id ){
	if ( !current_user_can( 'edit_user', $user_id ) ){
		return false;
	}
	// Reset the checkbox
	if( !isset( $_POST['wpmu']['guitar'] ) ){
		$_POST['wpmu']['guitar'] = 'No';
	}
	if( isset($_POST['wpmu']) ){
		foreach ( $_POST['wpmu'] as $key => $value ) {
			update_user_meta( $user_id, 'wpmu_' . $key, $value );
		}
	}
}