You can add a milestone from the Schedule page within your project (Project > Plan > Schedule). Adding a milestone follows the same flow as adding a task. You cannot rename the milestone from the Schedule page. If you need to rename it, do this from the work breakdown.
Expand the stage under which you want to add a milestone.
Hover over the stage name and click on the icon to add the milestone. The new field defaults to a task.
Click on the icon to change it to the milestone icon. Hover over the icon to see a tooltip.
Type the name of the milestone in the field and click out of the field to save it. If you press enter, it will add a new task below the current line.
Any changes you make will not be saved if you leave the milestone field blank and click out.