The Server page has a call out for servers with an out of date agent.
Clicking on this button will load a filter which includes all servers with agents older than the last two released agents.
Currently, upgrading the agent must be done on the box itself (it cannot be initiated from the UI).
Threat Stack has migrated the agent delivery over to our APT and YUM repositories. Upgrading can be managed using your distributions normal package upgrade mechanisms.
Additionally, you can leverage our Chef/Puppet/Salt/Ansible code available on Github to help you install/upgrade your agent.
https://github.com/threatstack/threatstack-formula (for Salt)