I would erase all of existence and then set about rebuilding it properly, starting from the background vacuum energy and working my way up. Sadly this new creation probably wouldn't include KMC and its posters.
...though at some point I'd probably decide it was all too hard, put it back the way it was, and hope nobody noticed
That would be my second choice.
You would run into one of those God paradoxes. Can you make a sandwich that is too big to eat? If not then you are not truly omnipotent with the cube, because there are limits to your power. But if you can then there are still limits to your power, since you aren't capable of eating it.
But as Kubik said, there are different levels of omnipotence.
It could be argued that it is a redundant question. If I were truly omnipotent I could bend the rules of the paradox itself. I could make the sandwich to big to possibly eat and still eat it, out of my omnipotence.
but... that would be cheating! Perhaps it could be done via a quantum sandwich, which simultaneously existed in both a state of being too big to eat and a state of having been eaten.
Well yeah sure it could be explained scientifically, philosophically or in any other way. But it would be explained away in accordance with the wishes of the omnipotent and his sandwich
i would just mull over the awesomely massive dump i would take just by eating half a sammich thats too big to eat...but remember when it comes to the cubes the more you use it and the grander the scale the quicker they evolve thier own sentience, so you'd have to get as much as you can out of it before they go