Go to 'Mobile' in the main toolbar > 'Settings' tab below the top toolbar. They are two main options:
Desktop view - Visitors will see your desktop site design when on a mobile.
Optimised view - A mobile optimised version of your site will be displayed when on mobile. This is created automatically and is a simplistic version of your desktop site. If you select this option, you can also choose to include a link to the desktop version, or prompt visitors to choose their view before entering the website.
Redirect to mobile site - If you have a mobile website hosted externally, add the URL to direct mobile visitors straight to the site.
Automatically choose mobile version for visitors
Show 'View standard version link - give the visitors the option to view this version
Give mobile visitors a choice of version - On the first visiting your website from a mobile device, visitors will be presented with a choice to see the desktop or mobile version and you can add a custom message.