I have created a simple web site for GitHub Pages. The source of the site is in the "master" branch and the generated web site (what I want to see published) is under the "gh-pages" branch.

[![Branches](https://i.stack.imgur.com/NQQpR.png)](https://i.stack.imgur.com/NQQpR.png)

I was expecting to be able to change the source of the site in the settings. However, the setting is grayed out? I cannot change it (see screenshot below). What am I doing wrong? How do I switch to the "gh-pages" branch?

[![enter image description here](https://i.stack.imgur.com/rwWY9.png)](https://i.stack.imgur.com/rwWY9.png)