If you forgot the password and would like to get back to default password you require to perform a factory default, but since there is not button for it, you need to shortcut pins:
a) while it's on the Digi Prototype (development) board:
With the Digi powered off connect pins 2 & 3 of the P12 pin header. Then power the Digi up and wait for about a minute. You can then power the device off, remove the connection from pins 2 & 3 and then power it back up. It should be back to factory defaults.
b) while it is soldered on your custom board (or not soldered at all):
1) power off
2) connect pin 20 (/init) to GND
3) power on while keeping pin 20 connected to GND, you might see an LED 1-1-1 blink pattern once.
4) after 30 seconds release the pin 20 to GND connection, you might see an LED 1-5-1 blink pattern once. Wait for the firmware to boot up with default settings
(the IP address will be reset to default which is get an IP from DHCP), so you will require the device discovery tool to find it's current IP
5) be aware that on the -UPW unique password devices, all chipers will require to be recalculated after a factory reset. This can take up to 40 minutes runtime and will keep the CPU busy, the device responding slow after next power on, until the keys are generated and stored. Login with password check or via SSH might not be possible at all until the keys have been re-generated.
to find pin 20 (/init) and pin 16 (GND to pins up), check the hardware reference manual: