...
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.
...
Using your web browser navigate http://dphone.dl.digium.com/ and choose the firmware version to be installed. If you don't specify a sub directory the phone will upgrade to the latest version available.
Once you located the firmware, on your phone, press the Settings softkey then choose Fetch Configuration file from URL (option 3).
Leave the protocol field at the default setting HTTP.
Move the cursor to the Server field by pressing the down arrow key.
The server field will display multiple boxes, change this to accept a hostname by pressing the second home key from the left [IP]/Host. Now enter the hostname: dphone.dl.digium.com/[x.x.x.x]
Move to the port field. This should be set to 80.
The path can be left at / (the root directory). If left at root, it will default to 000000000000.cfg which will download the latest firmware.
...