To upgrade MAAS from 2.4 to 2.5:
sudo apt upgrade
If you're running MAAS 2.3 or older, please see the following for more information about an upgrade path that makes sense for your installation. MAAS 2.3 was the last version to support Ubuntu 16.04 LTS.
If you're running MAAS 1.9, please start with the following and then see the section above. MAAS 1.9 was the last version to support Ubuntu 14.04 LTS.
See the Releases section on the Ubuntu wiki for more information about the Ubuntu release schedule.