Drive upgrades after Firmware 6.9.2

You can use 6 TB disks.

One approach is to power down, remove both disks, and install the two larger ones.  Then do a factory install (which will happen automatically at power up if the disks are blank), reconfigure the NAS and restore it from backup.

If you are running XRAID, the other approach is to hot-swap one disk, wait for the resync to complete, and then hot-swap the other.  After the second resync the system will need to be rebooted, and then the vertical expansion will start. The two old disks would probably end up out-of-sync, so you couldn't boot up with them both inserted, but you could boot up with either one of the them (keeping the other slot empty).

Sync times on the RN100 series are pretty slow, and the first approach only syncs once.  So the time difference between the two isn't as big as you might think (and in some cases it is quicker to do the reset and restore the data).  If your data volume was created a long time ago, you likely will see a performance improvement if you go with the reset - Netgear has made some adjustments to the BTRFS setup over the years, but those only kick in when you create a new volume.