...
Additionally, we would like to point out that firmware downgrades may not be possible in some cases. There are phones that will reject an old firmware version. This is usually done to address a major bug in the phone firmware code that has the potential to create significant issues.
Upgrading the phone using the web interface (Applies to all phones except D80)
Choose the firmware version to be installed. This can be found by visiting http://downloads.digium.com/pub/telephony/res_digium_phone/firmware.
Obtain the phone's IP address. On the phone's display select More > More > Menu > About.
Enter the IP address into your web browser, then enter the username 'admin' with the password '789'.
Select the General tab then choose Firmware Update.
...
Select the More softkey twice then choose the Menu softkey.
Navigate down to Advanced and select Reset to Factory Defaults.
Once the phone has booted back up you will then be able to use the default username and password listed above.
Upgrading the phone using XML from Digium Firmware Servers. (Apply to all phones except D80)
Before updating or downgrading the firmware Reset the phone to factory defaults.
...