From the main menu:
1. press "PHONE key
2. enter * 22 # and press "SPEAKER key" (you will hear 1 - 2 short confirmation tones)
3. enter old password/pin 0xxxx followed by # (so far the 0 and the extension 4-digit xxxx)
4. enter new password/pin (e.g. 23689536) (min. 5-digit, max. 8-digit) followed by #
5. Enter new password/pin (e.g. 23689536) again (confirmation) followed by #.
You will then hear a rapid sequence of beeps = change successful.
This only works for physical Phones, not for the Softphone.