For more information about publishing sources, see "About GitHub Pages."
Choosing a publishing source
Before you configure a publishing source, make sure the branch or folder you want to use as your publishing source already exists in your repository. For example, before you can publish your project site from the /docs folder on the master branch of your repository, you or a collaborator must create a /docs folder on the default master branch of your repository. 
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On GitHub Enterprise, navigate to your site's repository. 
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Under your repository name, click Settings.  
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Under "GitHub Pages", use the Source drop-down menu and select a publishing source.  
Troubleshooting publishing problems with your GitHub Pages site
Note: If your site's source files are located in the default publishing source—master for user and organization sites or gh-pages for project sites—but your site has not published automatically, make sure someone with admin permissions and a verified email address has pushed to the publishing source.
If you choose the docs folder on the master branch as your publishing source, then later remove the /docs folder from that branch in your repository, your site won't build and you'll get a page build error message for a missing /docs folder. For more information, see "Troubleshooting Jekyll build errors for GitHub Pages sites."