Hi Yu
Could I ask you to check that the browser has not zoomed the viewport which would cause the mobile menu to display. Using a PC you can do this by hitting 'CTRL + 0'.
To reduce the width at which the mobile menu kicks in which would resolve the issue you can do the following...
In your templates styles.php (../templates/j51_vitality/php/styles.php) cut the following code (line 32)...
/* Hide default hornav menu */
.hornav, #socialmedia {display:none !important;}
/* Show mobile hornav menu */
.slicknav_menu {display:block;}
Then open your templates responsive.css (../templates/j51_vitality/css/responsive.css) and paste the above code after the following (line 38)...
@media only screen and (max-width: 767px) {
Ciarán