Installing the Universal Plugin Manager
The Universal Plugin Manager (UPM) comes pre-installed in current versions of Atlassian host applications that use it, so you don't normally need to install the UPM manually. That said, if you do need to install the UPM on an application that doesn't already have it, you can download the UPM distribution JAR file from the UPM listing page on Atlassian Marketplace.
Make sure that the version of the UPM you attempt to install is supported for your application version, as indicated in the version history information on the UPM listing page on the Marketplace.
After downloading the JAR, you can install it to your application by copying it to your add-on installation directory, as described in the instructions for replacing the UPM JAR on disk in Updating the Universal Plugin Manager. However, instead of replacing the UPM JAR file, you simply copy the file to the specified directory.
Note that the UPM is a "Plugins 2" type add-on, so be sure to move the UPM JAR file to the directory indicated for that type of add-on.