Click on the requirement to view the details. Click on the current priority to the right of the requirement status to select a new priority from the dropdown menu. Choses are High, Medium, Low.
Note that changes to requirements may only be made when the requirement is in Draft status.
Users will need to have project permissions to view and edit the requirement to perform this action. 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 or reviewer will be able to view it.