Can I replace a steward in Hearthfire?

The Dismiss Steward mod allows you to dismiss stewards. I admit I haven't tried it yet but if you really want Lydia fired, I recommend making a save, downloading the mod, and giving it a try. Just make sure that you have a save from before you installed the mod just in case something catastrophic happens.

There is but one way and its to slay her, she cant go back to breezehome because her home has been changed to your other house, if you kill her, you can recruit new stewards in after her, but if there is a way to return her there, i dont know of it.