Epics can only belong to one Milestone at a time and they can be added to a Milestone in one of two ways:
1. Drag and drop Epics from the Epics Backlog on the Milestones page onto the corresponding Milestone card.
2. Click into the individual Milestone and select Epics from the drop-down. You can only select one new Epic at a time to add and Epics can be removed by de-selecting them in the list.