You remove the allen head bolts from the bottom of the corners.
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Adding to "Kenscollick": There are several allen head bolts on the underside of the unit. Those which have the same size as the ones in the four corners (remove the sikes first to get access to the bolts, have you?) have to be removed as well. Put the unit on one side and make sure that the top won't drop when loosening the last bolt! And make sure you put back on the top the same way it was before. It matters!
One last hint: don't put the whole thing together until you are REALLY sure your setting is ok. :-)