Measure the length of the user's lower legs while seated in the chair. Measure from the back of the user's heels to under their kneecaps while their legs are bent at a 90 degree angle.
Measure the chair's height from the floor to the seat of the chair. Compare this to the measurement of the user's lower legs. Decide how much you need to lower the seat by so that the user's legs will be at a comfortable 90 degrees and still comfortably clear the floor if the footrests are to be used.
Remove the seat cushion in the chair. This will lower the seat approximately half an inch.
Examine the chair and look for a fork in the frame with holes on each side where the caster and axle is secured. Adjust the caster to a higher hole in the fork on both the front and back of the chair, lowering the seat to a desired height.
Use your screwdriver or wrench to tighten up the caster and make sure all bolts are secure.