If the user knows their existing passcode and simply wants to change it for security purposes, this can be done by logging into voicemail as normal and choosing option 8.
However, if a user cannot remember their passcode, it will need to be reset by the company administrator. Process is as follows:-
Log in to the Horizon platform at https://www.unlimitedhorizon.co.uk/webapp/signin
Note that you must be logged in as the Administrator NOT a User.
You will see a screen similar to the one below.
Hover over 'Users' and click 'List Users'.
Click 'Edit' against the User whose Passcode you wish to reset.
Click 'Reset Passcode' at the bottom of the screen. This will generate an email to the selected User giving them a new Passcode to use. The user can then log into voicemail on their phone, at which point they will be prompted to change it using the keypad.