There are several options to upgrade NSM On-Prem firmware. The easiest one would be from NSM On-Prem Console CLI.
Note: this is for upgrading to latest firmware only. If current firmware cannot be upgraded to latest one, please follow upgrade path to upgrade to a version that support upgrading to latest version.
In NSM On-Prem Console CLI, type "?" to show all available commands. Note "?" is not visible:
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Type "update ?" it will show update options:
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Type "update check" it will return latest firmware version.
Type "update download" it will download latest firmware.
Type "update install" it will install the downloaded firmware.