Prusa i2, y-axis stepper motor calibration

I'm very new to 3d printing and was offered an 3Dprinter no one was using anymore.
From what i found in the wiki it must be a prusa i2.

I managed to hook it up to my laptop and when i connect with pronterface it says "Marlin 1.0.0", supose this is the firmware.

First thing I printed was a 20x20x20 cube. While it has perfect 20mm in x axis, the y axis is considerably short with only 17mm, height a little off with 19mm.

What are my options to correct this?

I read that one way is to reupload the firmware with adjusted values the motorsteps per unit for x,y,z,e.
But i'm not sure if there are other parameters i have to know about, which could get lost with a reupload. Is it possible to read the current values for x,y,z,e axis steps per unit from the firmware to correct them without reuploading the whole program?
Could it be a hardwareproblem (the printtable moves a little "stiff" in y-axis, otherwise everything seems ok to me)?

thanks a lot, every help appreciated

X and Y should be determined by the gear and belt used and stick with that. If it's off a lot the motor may skip steps. Look into the motor current adjustment (if you're using a board with stepsticks, there's a tiny potmeter you can adjust). Also see that the y carriage doesn't bind and the smooth rods are lubricated.
The same goes for the Z axis, the last half a mm you can adjust in steps per mm. threaded rods have a small percentage error that can't be fixed in hardware (unless you're using lead screws).

If you want to get your current values in MarLin send a M503 command to get your steps per mm. Check them with the Prusa Calculator based on printer hardware to see if theu match.

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Prusa i3 Rework - Ramps 1.4 - E3d Lite6 - Full Graphic LCD Controller