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 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”.