Change Default branch in gitlab

To change default branch in GitLab:
1. Settings > General > General project settings > Expand
2. Default Branch > Change your project default branch
3. Save changes

For gitlab v10+ (as of Sept 2018), this has moved to settings-> repository -> default branch

As stated by @Luke this is still valid as on 4/1/2021

in the GitLab Enterprise Edition 12.2.0-pre you have to use following: Setting → Repository → Default Branch ( expand it) and change the default branch Here

In 8.0+ it looks like this was moved into the project. If you open your project and go to the gear icon on the right, then "Edit Project" you can set the default branch for the project.