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.