Q: how do I change my su or sudo password in Yosemite

Hello klkittel,

Welcome to the Apple Support Communities! When using Terminal for sudo commands, you should be prompted for the admin account password. Since you know your regular admin password - you shouldn’t have any issues.

Mac OS X: sudo command requires a non-blank admin password - Apple Support

http://support.apple.com/en-us/HT202035

The sudo Terminal command can be used by administrators to execute commands as a different user (for example, as root). When executing this command, you will be prompted to enter the password for the administrator account you are currently logged in as.

Cheers,

Matt M.