In fact it's 2225022676 P-0213I.
You've got hardware revision I, and it came preflashed with the 3.2.4 versions if I understood well.
As for today, we don't know if HW-I can be downgraded to 3.1 versions.
Assuming it cas, you need an USB/TLL interface. They're often using an FTDI chip:
You then connect 3 cables to the following pins on the Dock/J01 port:
And you just send the 3.1 Boot2 image file with the HyperTerminal software included on Windows for example.