If you want to make a milestone out of an already created activity, you can easily convert the activity into a milestone.
To do this, select the activity and right-click to open the context menu. Then click on Convert to milestone.
This also works the other way around: select a milestone, right-click to open the context menu. Then click on Convert to activity.
Info Activities are displayed as bars and milestones are diamond shaped in the Gantt chart.
ATTENTION Summary activities cannot be converted!