The default priority of a bug is P0. To change the priority, click on the current priority to expand a dropdown, then select the desired priority to update. Indicators of the bug priority starts from P0 (high) through P3 (low).
Users with view and edit permissions in bugs can perform this action. Depending on the settings of the user's project permissions, they may be able to view all the project bugs or just a subset of them. Watchers specifically added to the bug will be able to view it.