O.K. - Got it figured out
1) Insert USB Stick into front panel USB socket
2) Put SDG into System / Info / Upgrade
3) Select telnet file for SDG2042X and UPDATE
4) LEAVE POWER ON SDG
5) On remote computer console, type: telnet <SDG IP address> <SPACE> 10101 <RETURN>
6) On remote computer console, type: mount -o remount,rw ubi2_0 <SPACE> /usr/bin/siglent/firmdata0
7) On remote computer console, type: cp /usr/bin/siglent/firmdata0/NSP_system_info.xml /usr/bin/siglent/firmdata0/NSP_system_info.xml.orig
On remote computer console, type: vi /usr/bin/siglent/firmdata0/NSP_system_info.xml
9) On remote computer console, move down to the line that starts with <license> and press 'i' for INSERT and right arrow over to just after </license> and press the delete button until the line only has </system_information>
10) On remoter computer console, press the ESC key, then type::wq <RETURN>
11) On remote computer console, type: sync
12) on SDG: remove USB stick
13) on SDG: turn power OFF
When you power back on, the status menu indicates the model number is now SDG2122X. You should see your serial number there, as well.
Thank you to JDubU and CustomEngineerer for your kind corrections!