You can use BackHub to export your data from GitHub. The full contents of the backup can be downloaded or exported via Cloud Sync to Amazon S3.
Export via Amazon S3
The easiest way to download all your data at once is to connect an Amazon S3 bucket via Cloud Sync and download all data using the S3 CLI or your preferred client.
Download GitHub repository including all branches
With BackHub you can easily download the complete repository including all its branches and git history. This is different from the download at GitHub, which contains only files of the branch you have currently selected.
To download the repository including all branches, follow the instructions here.
Another option is to clone the repository directly from BackHub servers.
Download GitHub repository metadata such as issues
Besides the repository itself, you can also download the metadata associated with it. The download comes as a ZIP file containing various folders for each type of data. Metadata is stored in JSON format.
Download GitHub wiki
If you have a wiki connected to your repository, you can download that as well.
Wikis are included in the backup of your GitHub repository. A GitHub wiki is simply a separate repository with markdown files containing your wiki pages.