At the moment, you don't. We have to have a custom kernel that supports it, first. But there are apps like CPURoot (I think that's what it's called) that allow you to drag a slider to change the frequency for those who do have one.
EDIT: I might need some brave testers soon

I'm making progress with the android app and have my dashboard and about screen finished, a working (basic!) file browser, and root privileges! All that's left is to add the actual commands which do not look hard. If I do release this, I doubt I'll put it on the market, it will remain free, and it will NOT have ads.