Many old firmware versions no longer available on the updated Siglent websites are still available here:
http://old.siglentamerica.com/gjjrj.aspx?id=15&page=1
Interesting... that old site is a bonanza of information
FAQ
Oscilloscope Hardware Reset
Solution: In some extreme hardware errors, the oscilloscope may become
unresponsive or “hang” during the boot sequence.
If your oscilloscope is frozen, will not boot, or appears to be unresponsive, you can
attempt a hardware reset by following these instructions:
1. Disconnect all USB, LAN, and input cables. Leave the power cord connected.
2. Power cycle the instrument by pressing the power button.
3. Immediately begin pressing the MATH button repeatedly at a rate of 2 to 3
times a second while the instrument boots up.
4. If the instrument does not boot up properly, retry steps 2 through 4.
NOTE: If you continue to have problems, contact your nearest SIGLENT
office for service.