Well, you need to attach the probe to that contact; it outputs a signal (typically a square wave) at a certain amplitude and frequency (check your manual). Set your scope controls for that amplitude and frequency (voltage/time-base), and see if it matches what your manual says. If it does, all good. If it doesn't - then you typically tweak various calibration controls on the scope front (generally recessed, and you might want to use non-conductive screwdrivers for the adjustment).
Most of the time, it will only need very minor adjustments, but if it is really off and the adjustment controls on the scope don't help, then you need to get it professionally calibrated by the manufacturer's service center (which may not be cheap); they have special calibration signal sources they use for this work, plus they can tweak interior controls and such (or I imagine for newer scopes they adjust things in software)...
And don't even think about doing it yourself - I looked into what it would cost for the calibrated signal source for one of my scopes after I got the service manual for it that gave calibration instructions - the cost for that source tool was INSANE - then I wondered about how much it would cost to get the calibrated signal source device calibrated...
Welcome to the wonderful world of scopes - nowadays, its probably almost cheaper to buy a new scope, than to get the old one calibrated...