There's a set screw on the barrel of the arm. You loosen the screw, move the arm up or down, and retighten.
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You can place a stack of playing cards in the gap between the arm mounting collar and the large black platform to shim up the arm and retain the current VTA setting once the setscrew is loosened. Otherwise, the whole arm will drop down to the armboard and you'll be back to square-one with no height calibration reference other than a few red index dots marked on the arm pillar.
Add or subtract one card at a time to make fine, repeatable height adjustments and note the number of cards used for future reference.
I've found that it's important not to overtighten the setscrew. Just give it a gentle 1/10 turn past the point where it contacts the arm pillar.