Please follow the steps below in order to update your sandbox to work with forked edx-platform repository:

• Create sandbox in usual way

• SSH into your sandbox – ssh <your-github-username>@<sandbox-dns>.sandbox.edx.org
• sudo rm -rf /edx/app/edxapp/edx-platform – to change the remote you need to remove the existing edx-platform checkout
• Add the following line to /edx/app/edx_ansible/server-vars.yml - edx_platform_repo: "https://github.com/<user>/edx-platform.git"
• sudo /edx/bin/update edx-platform <branch-name> - update edx-platform using the forked repo