NPM 8.5 is legacy and obsolete, upgrading that isn't an easy job. The best approach would be to install the latest version on a new box and yes you will need an active SolarWinds license to do it.
Install it on a new server and decommission the existing one.
Plus you wont be able to use 'SolarWinds Product Upgrade Advisor' and the version that you are running on won't be visible on that either. (As mentioned earlier NPM 8X is obsolete)
I'd suggest to go to SOLARWINDS PRODUCT UPGRADE ADVISOR at https://customerportal.solarwinds.com/support/product-upgrade-advisor
and select relevant path of upgrade Orion platform as well as OS-es and database.
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