There are several methods by which you can reset your voicemail passcode.
1. Dial *2 and follow the prompts to reset your passcode.
2. Go to www.sprint.com and click on My Sprint from the top banner. From My preferences > All about my account > Things I can manage online, select the option Change Voicemail passcodes.
For further Visual Voicemail support, go to the customer care page on your service provider's web site.