Best bit of a thread at: 
First you must be sure it is out of calibration and not simply a poor seal, which usually (nearly always) is the cause of this symptom. To ascertain this, you must remove the white shaft by removing the tip ejector (it pulls off), then unscrewing the blue shaft coupling. Remove this carefully over a clean bench as some parts may fall out and get lost if you are not careful. Unless you have a cache of spare parts (a wise idea, but it sounds like you are a novice with these devices), such a loss will cripple you until an order can be transacted with Ranin (Gilson distributor in the USA). It would be helpful if you have the little booklet that Gilson ships with pipetmen, as it contains an exploded view of the insides. At any rate, remove the O-ring/teflon seal combination from the shaft and then remove the part above it on the shaft. Then reinstall the O-ring/seal combo and see how much it clearance it has as you slide it up and down on the shaft. If it is snug and cannot shimmy or wiggle and will not fall off it oriented vertically, then it is OK. Otherwise, you must place an order with Ranin (Mack Road, Woburn, MA, if in the US) for the seal for your size pipetman. I recommend a sizable number of these (aside from minimum order requirements) as we wear them out every few months. DO not replace the O-ring, as they never go bad.
If the seal is OK and it is still pipetting less than the setting, then it may need recalibrating. Actually this is a misnomer, since the "calibration" of the pipetman is fixed by the pitch of the micrometer threads and can never be "out of calibration". What can change and be changed is the limit on the upper end of the travel of the plunger (the one with the button on it that your thumb touches). The other thing that can be changed (by mistake) is the proper remounting of the pair of knurled knobs that flank the rubber "Friction ring" - but only if it was removed to replace the friction ring (which crack and fall off after a few years - another worthwhile spare part to get.) Assuming this is not the problem, then to see if recalibration really is needed, determine the error of the pipetman at each of three or four settings equally spaced across the range it is capable of measuring. Recalibration is called for if the error is constant (or nearly so) in volume. In your case, the amount delivered would be expected to be 5 ul low for all settings if it relly needs recalibration. As you will see below, it is exceedingly unlikely that recalibration is needed, because the adjustment cannot become loose.
Recalibration (changing the limit on the upper travel of the shaft) is done by using an allen (hex head) wrench to loosen the three or four set screws in the TOP knurled knob (above the friction ring). Once loose, a small flat end screw driver can be used to move the metallic colored slotted collar around the shaft. It is threaded and has a very fine pitch, so more than one turn may be needed. With the proper tool this would be easier to do. With each adjustment, try out the pipetman again to see if the correct setting was obtained. When obtained, tighten the set screws and check the calibration at each of several volumes. It should now be correct if this was the problem in the first place.
If you need help, call Ranin (if in the US) for tech help over the phone. I do not recommend sending it back for service, as they usually tell you it is too badly worn to repair (not true) and tell you to buy a new one. For such bad news they charge a small fee.
Good luck. Write if this cannot be interpreted or understood.
Curt Ashendel Dept. Med Chem., Purdue University, W. Lafayette, IN Ashendel at aclcb.purdue.edu