Click on the requirement to view the details. The requirement type is located on the right side of the details window. Click the current requirement type to expand the dropdown and select the desired requirement type. The dropdown also has a search bar available for quicker selection.
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, reviewer or co-owner will be able to view it.