How do I change the root password?
If you can already log in on an account with administrator privileges (normal OS X account) and know its login password, you can reset the passwords of other admin accounts in System Preferences > Accounts. If you can't, you can reset the passwords of admin accounts in single user mode. See this question.

If you actually mean the password of the root user, it can be changed from /System/Library/CoreServices/Directory Utility.app/, but you have to be logged in on an admin account and know its login password.