No “installation” of nssm is needed. Just place it somewhere on the system (preferably somewhere in your PATH) and run it.
Note however that nssm registers itself as an Event Log message source which means that running multiple instances or different version of nssm from different locations may cause confusion. Also note that if you run the Event Viewer it will open the nssm executable, preventing you from overwriting it. Keep this in mind if you come to upgrade nssm.
Some features are labelled as supported as of a particular version. If the version described is newer than that available from the download page there may be a pre-release build with the feature enabled.
Equivalent command examples below show the commands which would configure an existing service to match the screenshots. In many cases they represent the defaults for the parameters in question and are thus redundant. Any parameter can also be reset to its default value with