Well, Looks like Nokia N86 also has got a firmware update to v21.006 from its previous version v20.115.
It seems like this is just a maintenance release aimed at improving the performance of the device and fixing all the bugs in the previous firmware with no special features attached to it.
Although the Nokia N86 supports User Data Preservation (UDP), we highly recommend you to take a backup of all your data before kickstarting the firmware update to prevent any data loss.
You can update the firmware of the phone either by Over the Air (OTA) on the device or Nokia Software Updater (NSU) on the PC.
If your device does not detect any new firmware, then relax, the update might not have reached your region product code yet. The firmware update will eventually roll out to all product codes of Nokia N86.
As this is a maintenance release, there has been no specific changelog for this firmware release as yet, but we will update you as soon as we get one.
Also, if you find anything new in this firmware release, do let us know via comments below.