The default version of a model is the version that is used when an action is taken against the model as a whole. For example, if one opens a model but not a specific version, then the default version is opened.
Steps
1. Sign in as a user with at least the Metadata Management capability object role assignment on the model.
2. Go to MANAGE > Repository.
Be sure the Repository Panel is filtered to enable showing of versions.
3. Right-click on the version of the model in the Repository Panel and select Set to default.
The previous default version will no longer be default.