use a drywall sanding pole. But this is only to knock of the high points and some surface cleaning. To make a better finish you may need to skim coat, sand, prime, touch up the missed spots, prime and two coats of quality paint.
You could use a powered drywall sander (Porter Cable makes one, for example) IF you could find somewhere that rents them over there. It's still just going to knock off the high stuff though, and the sandpaper (no matter what system you use) will "gum up" from the paint pretty quickly. You'll still need to use joint compound on the really bad areas as Bob stated....
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