If you know your existing passcode and simply want to change it for security purposes, then you can do this by logging into voicemail as normal and choosing option 8.
However, if you cannot remember your passcode, it will need to be reset by the company administrator. Process 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.