You can find the Restart Server button on the Administration | Diagnostics page in the TeamCity web UI. TeamCity will also offer you to restart the server if you upload or disable a plugin on the Administration | Plugins page – the Restart server to apply the changes button will appear at the top of the page. It is also possible to restart the TeamCity server from the command line – stop and then start it as described in this article.
During the server restart, agents will continue running the builds that have already started and will upload the results of these builds to the server once it comes back online.