By default, the Autoupdate and install option is enabled for the On-Site Agent (On-Site Agent Registrator > Options > Autoupdate), and the agent version is updated automatically.
Automatic updates are disabled if you choose to activate an older version of the On-Site Agent application on your machine.
If the Autoupdate and install option is not enabled, follow the instructions from How to Manually Update On-Site Agent Version to update the version manually.