Do you have to be the repo owner?
Yes you do.
How to set the default branch on github.com when I go to browse commits.
If you fork the repository, you can change the default branch of your fork, by clicking on the Settings button (see below).
This will allow you to pick your favorite branch as the default one for your own fork, but you’ll have to keep your forked repository in sync with the upstream repository by yourself.
Or you can directly jump to the following url https://github.com/{:user}/{:repo}/settings