WP in /blog folder of existing site: how to make blog logo link to main site

I am installing a wordpress blog into the /blog folder of an existing PHP website. Eventually I will host the whole site in WordPress as a CMS, but for now my clients just want a blog quickly added to the current website.

How can I make the blog’s main title/logo link to website.com instead of website.com/blog so that it works more like a properly integrated blog should?