In the bug list in Milestones, check the box(es) to the left of the bug code and select Move bugs 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 bug(s). Note that bugs 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 bugs.
Users will need view permissions in bugs to perform this action. Additionally, depending on a user's project permission settings, they may be able to see all the project bugs or just a subset of them.