An API version will have one or more branches. It will have exactly one default branch. The default branch may be changed at any time.
The default branch is set when you connect your source. If you use a Git-based source, it will be your Git branch name. If you use the file upload source, you will be asked to pick a branch name. If you use the URL source, you will receive a default branch name, because this source does not support branches.
You may change the default branch to any other active branch. If you want to change to another non-active branch, you will need to edit the source.
To change the default branch to a non-active branch, you will need to edit the source access at the version overview or settings screen.
To set it to another active branch, go to the version's Settings > Branches screen and select it.
The branches that display in other active branches depends on your source settings.
If you selected "Validate and bundle from other branches", then any subsequent push to a branch will trigger it to be added to the other active branches.
If you selected "Validate and bundle from PRs", then any subsequent pull request (PR) will trigger it to be added to the other active branches.
If you delete the branch in your connected source (for example, GitHub), it will be deleted from the other active branches. Otherwise, you may delete it by clicking on the trash icon from Settings > Branches.