New to NAS

I want to upgrade my two raid disks, on mycloudultra2 can I do this 1 at a time or must they both be done at the same time

Well, depends on how you want to get the data on the new disks.

If you are copying from another computer or NAS; you can do both at the same time. Just insert the disks and create a new Raid.

If you want to copy from the older disks. . .it gets trickier.
1) You really should back everything up first before messing with this.
2) If you put in a fresh disk in slot 2, it will copy everything from the original disk in slot 1 to the new disk in slot 2.
3) You can then repeat the process by putting a new drive in slot 1, and the system will copy everything from the new disk in Slot two over to the new disk in slot 1.
4) IMPORTANT: There is a nuance if the new disks are BIGGER than the old disks; when you have an old disk and a new disk in the machine, I believe it will configure the new disk to be the same size as the old disk. Once you have two (larger) new disks, there is an option to make the “volume” bigger. I HAVE NOT DONE THIS PROCESS, so you may want to get advice from someone who has for the details.
(Obviously; you don’t want the new disks to be “smaller” than the old disks).
5) NOTE: The WD software configures the drives as Linux devices. If you put the old disks in a PC or MAC; it won’t directly read the disk unless you get some software to read the drive.

If you just want to use larger drive, the easiest method is to back up the current NAS files (eg to a USB drive), replace the two drives with the new larger drives and copy the USB drive contents back to the new NAS.