Click on the requirement to open the details window. To view changes between versions, there needs to be multiple versions of a requirement. In the Version Changes tab, select the From and To versions to view changes. Use the scroll bar on the bottom of the window to view changes over time.
Users must have project permissions to view 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. Users added to a specific requirement as a contributor or reviewer will be able to view it.