For the latest Roon Server and client software versions, you can always look at:
From this, you can see that the version 2.65 that you are running is latest production Roon Server software (2.65 - build 1653). It does not include the versions of RoonOS that may be running on ROCK installs and Nucleus devices.
It is likely that you are running the latest version of RoonOS for your ROCK installation.
Version 1.0 build 259 of RoonOS is last version that supports BIOS boot devices so if you originally installed ROCK prior to the first build to support UEFI boot (254?) or you chose to use BIOS boot for any reason, then build 259 is the latest one available to you.
There are newer builds of RoonOS (that still run the same version of the Roon Server software) but these do not support BIOS boot and can not be installed as an upgrade to the earlier versions. At present, the only advantage to the newest version 2.1 (build 271) of RoonOS is that it includes support for some additional HDMI hardware variants (courtesy of a fix for the Nucleus Titan) and it includes support for running Tailscale on the Roon Server machine.
If you are currently using BIOS boot (selecting ‘Reinstall’ does not upgrade you to build 271 of RoonOS), then the only way to upgrade to version 2.1 build 271 is to set your NUC to use UEFI boot (in the BIOS settings - at which point your NUC will no longer boot) and then reinstall using the ROCK installer package and an USB stick as you did when you first installed ROCK (and then using the ‘Reinstall’ button to bring the new installation up to date). This involves completely reformatting your system SSD in your NUC and so a backup to a USB disk or a network device must be performed first and, after upgrading to build 271 this backup must be restored to get your database and settings back.