In the External Items tab in Milestones, check the box(es) for the Jira issue(s) that need to be moved. Click the Action button and select Move external items to another milestone in the dropdown. Select the desired milestone and click the Go button, then click Yes in the confirmation window to move the issues(s).
Note that Jira issues can only be moved to a milestone which has a status of Upcoming or In Progress. Milestones with a status of Completed or Overdue will not appear in the dropdown as options for moving Jira issues.
Users will need view permissions in the tasks module to perform this action. Additionally, depending on a user's project permission settings, they may be able to see all the project tasks and external items or just a subset of them.