There are a couple of ways to add features to a release. Once features have been added, use the Wizeline to separate prioritized features (above the line) from un-prioritized features (below the line).
Add a single feature to a release
To add an existing feature to a release, select the feature from your backlog and click “Edit.” From the dropdown on the feature, select the release — or releases — to which you’d like to add the feature.
Add multiple features to a release
To add multiple features to a release, go to Features and select “backlog.” Select your desired features and then, using the Bulk Edit dropdown, select the release(s) to which you’d like to add your selected features. Don't forget to click “Save” on the upper right.