It should be really easy if you own a pair of calipers.
You print the test part, measure it, do the math problem to adjust your steps per mm, flash the updated firmware, print the part again and verify it worked.
But 99% of the time it is an extrusion issue. In your case I would bet it is feeding too much filament. Heat the printer up. Mark 100mm on the cold side from a fixed point, tell it to extrude 100mm I bet it is going past your mark. adjust the steps per mm upload and check.
lots of vids on this if you don't understand my simplified steps. If you have a printer best to take the 10 minutes and calibrate it.