In the requirement details window, hover the mouse over the current title until an edit icon appears. Click on it and edit the title. When complete, click the check icon or press enter to save changes. To cancel edits, click the or click outside the edit area.
A user must have view and edit permissions in requirements to edit requirements. Depending on the settings of the user's project permissions, they may be able to view all the project requirements or just a subset of them. Users added to a specific requirement as a contributor, reviewer or co-owner will be able to view it.