By default, the Extra Theme logo links back to the homepage. But this can easily be changed with the following jQuery snippet:

You can add this to the theme by pasting it into "Extra > Theme Options > Integration > Add code to the head of your blog", after anything else that is in there.

Just change https://www.google.com to your desired URL.

Divi Booster supports changing the Extra logo link (v2.6.5 onwards). You can set the link URL on the Divi Booster settings page at “Header > Main > Change logo link URL”.