If it's a single image (1 big tile) you can use the nudge tool. If it's a tiled document you can only move in tile increments. (You can also use the marquee selection tool but I wouldn't recommend it because it's not fully implemented yet.)
Thanks for the reply, I meant "re-ordering" layers though. The question wasn't very clear. I figured it out though, I used CTRL+Left Click to drag them around.
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(You can also use the marquee selection tool but I wouldn't recommend it because it's not fully implemented yet.)