上

How it works

を (object) marks 上(うえ) (up, above) as the thing the looking is aimed at — Japanese treats the direction of your gaze as an object here. The verb 見る(みる) (look) appears in its て (te-form), which on its own works as a casual instruction: "look up." There's no polite ください (please), so it's the relaxed version a friend would use.