You can add an existing issue to a Release by 2 ways:

Using Planning Board: Open the Issue Backlog in one panel and Release Backlog in another panel. Drag and drop issues to add them in selected release.

Using Issue Tracker: You can search for an Issue or Issues on the board and Bulk edit them by adding Release version in Release field.

Using Release Board: You can add an existing Issue by using drop down > Issue – existing. It will open Planning Board where you can search Issue. You can open different panels on this board. Open the Sprint/Issue Backlog in one panel and Release in another panel. You can drag and drop selected items to Release Panel.