Preferences > Beta options

[ ] I would like to beta test features for reddit (by enabling you'll be subscribed to /r/beta automatically. details on the /r/beta wiki)

Wait for page to reload, scroll down, then click the new option you'll see:

[ ] Use the redesign as my default experience (by enabling this, you will be redirected to the new site when you go to any supported https://reddit.com page)

If you enable beta-features, there's a setting for it in preferences.