The F2 key will get you into the BIOS. It identifies itself as InsydeH20 Setup Utility. Having updated the BIOS from version 1.0A to 1.0D I found it wanted a password. The usual Wyse password of Fireport worked.
When powering up the F9 will bring up a boot menu allowing you to select a pen drive if you have one plugged in.
There's not a great deal you can tweak in the BIOS compared to some.
Under the Advanced tab there are entries for HDD and SSD that can be enabled or disabled. From this I assume that you can fit an mSATA(?) SSD into the expansion slot - I don't have one to hand to check this.
Under the Boot tab the Legacy option lets you play with the boot order. The Hard Disk Drive option let's you put a USB pen drive ahead of the internal Flash - but it only appears in the list if it's plugged in when you enter the BIOS. Using F9 is an easier way for a one-off boot from a pen drive.
I subsequently got a second X90 for a few pounds (X90C7). This one came with an unknown BIOS password set and no flash, RAM or PSU. When I finally got around to the trying to bypass/remove the supervisor password I wasted some time as I thought, being a reasonably modern system, that the password mechanism would have a degree of sophistication to it. It doesn't. The password is stored in plain text in the CMOS/NVRAM. Assuming you can boot Linux from a pen drive (or the main flash) it's just a matter dumping the contents of the NVRAM.....
tc@box:~$ sudo cat /dev/nvram | hexdump -C 00000000 80 00 00 bf be fd 0e 80 02 00 3c fd f5 ff ef ff |..........<.....| 00000010 bd ff ff ff b7 ff bf fb ff ff ff ff fe b7 ff ff |................| 00000020 16 fe 00 3c 20 df ff ff fe ff f9 ff ff f9 ff ff |...< ...........| 00000030 ff fb ff ff 7f bf 01 fe fd df fd 7f ff fe ff ae |................| 00000040 ff ff bf ff ff df ff fe fd ef f7 f7 67 bf e7 ff |............g...| 00000050 fd 00 7f ff f4 ff d7 ff 7f fd fb ff a5 ff ff ff |................| 00000060 6e 01 09 72 65 64 73 74 72 31 70 65 5d ff 70 07 |n..redstr1pe].p.| 00000070 24 01 |$.| 00000072
...to be able to see the password which turned out to be redstr1pe in this case. (Maybe the previous owner came from/holidayed in Jamaica?)
The X90cw runs Windows Embedded Standard - essentially Windows XP. The one I got had had the flash memory wiped clean. Normally I'd have ignored this but this time I decided to restore the standard firmware.
A quick visit to Wyse's support pages found the necessary files - which incidentally included an update to the BIOS. It was a matter of moments to download the file and unpack it. I decided to use the stand-alone Wyse USB imaging tool to get it onto the X90cw. This took some time....
The Wyse support site has the two latest versions of the Wyse USB Firmware Tool - versions 2.0.9.15 and 2.1.0.12 as I type this. These do NOT work for older Wyse hardware such as the X90cw. (It happily prepared a USB pen drive, but that wouldn't boot - it just hung after printing the SYSLINUX opening line). Stuff I found with Google suggested that I needed version 1.5. I spent quite some time searching around for it but to no avail. Finally, checking my own Wyse folder I discovered that I had copies of v1.4 and v1.8. Version 1.8 did eventually run on my Windows 10 system after I played with the compatibility settings, but it did then end up taking an age to format the pen drive and copy the files to it. (Could it have been an hour compared to the few minutes that the more recent version took?).
Anyway the pen drive booted quite happily and restored the OS to the flash drive and updated the BIOS.
I believe older versions of the tool can be requested by email from the Dell/Wyse support people. I didn't try this as I took the pessimistic view that they're only interested in corporate clients with support contracts. I could be wrong....but what's the rational behind removing the tools that go with older products? The firmware itself is still available for download.
The next problem I had was the inability to login as Administrator thanks to the short-sightedness(?) of somebody at Wyse. With this combination of OS and hardware the usual Windows default administrator password of Administrator had been replaced by Wyse#123. This did not work for me until the penny finally dropped. The newly installed firmware is set up for a US Keyboard, not the UK keyboard that I had. Every time I thought I was typing the password I was actually typing Wyse/123 which didn't work. With the previous default password - all letters - it would have. (But thinking about it I guess French, Canadians and others with AZERTY keyboards might still be stuffed??). Anyway Wyse£123 got me in.
NB When using the control panel to change the keyboard language remember to tick the box 'for all users'. If you don't you'll start off with an assumed US keyboard layout until you log in.