What @blinky465 writes above should be the first step.
You may also check in the menu (or your slicer) if there is no some Z-height offset.
Have you printed anything apart from demo print? It always prints it in the very centre of the bed, you may want to try slicing something to be printed in the corners.
My I3 mega (I had it in 2017) had warped bed it that very way, that the central part was almost a millimetre higher than the corners. There is no easy way of fixing that other than replacing whole bed altogether. In my case anycubic sent me replacement from China after I sent them this photo:
EDIT: one more thing. Check if your X-axis rails are perfectly horizontal. This machine has rather poorly designed micro-switches turning Z-axis motors off when It reaches lowest point. They sometimes stop each motor at different height casing those X-axis rails to move not parallel to the bed plane.
