Then, you'll navigate to OctoPrint Settings by clicking on the wrench icon at the top of your OctoPrint browser window.
In OctoPrint Settings, click on Plugin Manager in the left navigation menu. In the Plugin Manager, click on +Get More.
This will open the Install new Plugins... window. In the search box, type in "mqtt". The MQTT plugin will appear at the top of the list. Click Install to install the plugin.
After the installation finishes, a Plugin management log dialog box will appear letting you know that you need to restart OctoPrint to complete the installation. Click Restart now to restart OctoPrint. When you're asked if you're sure that you want to restart, click Proceed.
After OctoPrint finishes restarting, you'll be prompted to reload the server. To do this, click Reload now.
After reloading the server, navigate back to OctoPrint Settings and the Plugin Manager. In the manager, search for "mqtt". The MQTT plugin should now appear under Installed Plugins.