Dial your extension or the extension of the voicemail you want to change. Let it ring and wait for the voicemail. Do not answer the phone call.
When voicemail picks up, then press the * key
Wait a little and it should play the name of the mailbox owner or greeting.
Enter your PIN then press the # key. By default the pin is 12 + 4 digit extension unless you've changed it. For example, 123456 for extension 3456.
You should now be able to change your voicemail.