Locate the leg that's loose and identify the black plastic cover over the adjustment nut.
Carefully pry off the cover with a thin screwdriver.
Set the cover aside so you can replace it later.
Unlock the clamp lever
Tighten the brass nut an 1/8 of a turn or less, close the locking lever, test if the leg no longer slips. If it does, loosen the lever, then tighten the nut again and re-test until the leg is secure.