Unlinking a task from a milestone can be done in multiple places.
In the Milestones tab, navigate to the Tasks tab and click on the vertical menu for the desired task. Select Remove and click Yes on the following confirmation window. Alternatively in the task list in Milestones, check the box to the left of the task code and select Remove tasks from milestone in the Action dropdown.
In the Tasks tab, select the task to view the details. Hover over the milestone code and click when it appears. Click Yes on the following confirmation window.
Users will need view permissions in tasks to perform this action. Additionally, depending on a user's project permission settings, they may be able to see all the project tasks or just a subset of them.