Latest is CodeWarrior for MCUs 6.1 which includes the HC(S) family. I don't think you can upgrade straight from HC(S)08 5.1 direct to CW for MCUs 6.1. That's a major version step, with a lot of things changing.
Best bet: Fire up the CodeWarrior Updater application and let it find out what patches and updates are applicable to your version of CodeWarrior. Usually this app is installed on Start menu, like CodeWarrior itself. So, you'd pick:
Start > Programs > Freescale CodeWarrior > CW for HC12 V4.5 > CodeWarrior Updater
---Tom