I don't believe that's available with the current website builder as it's actually one responsive site, meaning the elements are formatted to adjust to the screen size instead of having separate dedicated sites for desktop and mobile.
If you're not pleased with the mobile version of the site you've created, you can try several options for improvement. If your titles are long, you may wish to try a different font set that would work better on the mobile site. You may also try different templates without re-doing the website. May sure you publish the current version before trying new styles, so you'll have this current version to restore, if you wan to go back.
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