The Completed Milestone view lists milestones that are completed in a project.

Click Mark As Complete automatically moves your milestone under completed milestones.
In addition, it includes links to Archive your milestones. Once your milestone gets completed click on the checkbox next to it and your current milestone automatically moves under completed milestone. Furthermore, if you wish you can simply uncheck your milestone so it will reverted back to Upcoming milestone list.

Upcoming Milestones | Archive Milestones | Overdue Milestones