Unlinking a bug from a milestone can be done in multiple places.
In the Milestones tab, navigate to the Bugs tab and click the vertical menu for the desired bug. Select Remove and click Yes on the following confirmation window. Alternatively in the bug list in Milestones, check the box to the left of the bug ID and select Remove bugs from milestone in the dropdown.
In the Bugs tab, select the bug 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 bugs to perform this action. Additionally, depending on a user's project permission settings, they may be able to see all the project bugs or just a subset of them.