In the task list in Milestones, check the box(es) to the left of the task code and select Move tasks to another milestone in the Action dropdown. Select the desired milestone and click the Go button, then click Yes in the confirmation window to move the task(s). Note that tasks 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 tasks.
Users will need view permissions in tasks to perform this action. Additionally, depending on a user's project permission settings, they may be able to see all the project tasks or just a subset of them.