Requirement: Change Subsite logo to link root site in SharePoint 2013 / SharePoint 2016
Solution: By default, the Subsite logo links points to its homepage. We can change the site logo link in SharePoint 2013 using jQuery to point to the root site.
Change SharePoint 2013 logo link to root site:
Add a script editor or modify your master page to include this code
Here is another post with a different approach: Change SharePoint 2013/2010 Site Logo Link to Root Site URL