How do I change my VOIP voicemail passcode/PIN?

To Change your VOIP Voicemail Passcode/PIN Using Your VOIP Phone

Call into your VOIP Voicemail box.

At the Main menu, select the option Setup Options, then Preferences, and then PIN.

Follow the prompts to enter and confirm a new PIN. Use digits 0 through 9.

Once you are finished following the prompts, hang up.  

Phone UserGuide: 

https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/connection/11x/user/guide/phone/b_11xcucugphone.html