Final versions of plugins should be generally available on the Available Plugins tab of the Plugin Manager and easily installable. In cases you only have a local .nbm file of a plugin, maybe you got a preview version of a new processor or reader, the following steps explain how to install this plugin to your SNAP installation.
Start SNAP Desktop. Open the Plugin Manager. From the menu select Tools --> Plugins
Go to the Downloaded tab and click on the button Add Plugins...
Find in the now showing file chooser the .nbm file on your local file system and click on Open.
The chosen plugin will be shown in the list on the left side and a description along with some metadata will be shown on the right side.
The plugin is marked for installation by default.
Click on the install button to proceed with the installation.
During the following installation it might happen that you see a warning saying the the plugin is not signed. Usually this is not a problem but you have to decide your self if you trust the provider of the plugin file.
At the end you are asked to restart SNAP Desktop.
After clicking on Finish SNAP will exit, finalise the installation and restart SNAP Desktop again.