Ah, well a lot of people miss it. But, we like to keep structure to all our screens so now you'll know that the edit/delete/cancel button are always at the bottom of the screen. You'll never have to learn that again.
You can't revert it to draft (there are legal things there so we stop you from doing that) but, if you look to the right menu you'll see 'Copy Invoice', that will copy this invoice and leave it in draft. So just click "copy invoice', and then 'save', on the next screen. That's it.