It depends on the Boot method. ROCK installations using BIOS boot will only update as far as build 259. By contrast, ROCK installations using UEFI boot will update to the latest version of RoonOS whatever that might be (currently build 271).
Roon have suggested that a solution may be comming at some date for the Bios Boot Nucleus devices so such a solution would presumably also apply to BIOS boot ROCK installations. However it is not here yet:
Unfortuantely, if your ROCK install is using Bios Boot, there is no easy way of converting to UEFI boot. If your NUC supports it, you have to change the boot mechanism in the BIOS and then re-install ROCK from scatch (obviously making sure that you have an up to date, and valid, database backup to restore from).
Modern NUCs (I don’t know exactly how modern but it certainly applies to my NUC11) default to UEFI boot so a ROCK install performed recently on up to date hardware will almost certainly install RoonOS build 271 (or at least a version that allows you to use the “reinstall” feature of the RoonOS WebUI to bring the RoonOS and Roon Server installation up to date.