Once the milestone is created, it appears as a blue vertical bar in the timeline. By default, the milestone is created in the middle of current date. You can drag the blue bar to the appropriate date and the changes will be saved when the plan is saved.
It is possible to edit the milestone name or delete a milestone, using the toolbar highlighted on the right side of the blue bar, when you click on it.
Delete operation will ask for a confirmation whereas the Edit operation will show the dialog to update the current name.
Note that the milestones are specific to a plan and not a filter. It is possible to have multiple plans, against the same filter, with different milestones defined on them.