There's 2 things you can do. First is take off the side cover by the panel and tighten the belt by loosening the screws holding the motor with it and pulling back a little and then tightening the screws. See if that makes any improvement, if not then you can do a calibration with 400mm square. On the panel, use Function, cycle to #Adjust, then cycle to $400mm square. Oh, prior to this, you can change out the blade with a pen or you can still use the blade, but the pen is easier to see. And load a blank piece of media so the cutter can plot the square. Back to the machine, once you have this ready, tell it to draw and with make a half square and an A and B side. Pull the sheet off and using the mm side of a ruler, measure the length of the A and B lines. They should be 40mm each, but will end of being either short or long. The panel will have input the adjustments for each line by adding or reducing the length. You kinda have to fiddle with these and do a couple of draws to get it corrected. You can also adjust the angle of the corner to make sure its square, but usually this isn't a problem and I wouldn't worry about it.
