Control Panel\All Control Panel Items\User Accounts -> Manage another account -> Change the password.
WARNING: This works for local accounts. For Microsoft Accounts you'll need to use the https://login.live.com/
web interface, and......
Before you decide to make too many adjustments to user account passwords, make absolutely sure that you have one local account that is a member of the administrators group as a fall back, in case things don't work out.
We've heard any number of problems with user account passwords resulting in in-accessible computers.