The most likely cause for slanting prints is usually that the set screw in a pulley has come loose. This most often occurs during transit. Check the set screws on all of your pulleys, particularly the ones attached to the short motor belts. The 2mm Allen Key that arrived with your machine should fit in the pulley set screws. If the lean is left to right, rather than front to back, I'd pay particular attention to the rear most pulley on the lower right hand rod.
If you cannot see/access the set screw, manually move the print head until the pulley rotates so that the screw is visible.
The other thing you can check, if all of the pulleys on the rods are secure, is to tell the printer to home the head using Maintenance --> Advanced --> Home Head for Ultimaker 2 series printers, or System --> Print Head --> Home on the Ultimaker 3 series printers. If you start with the printhead at the front right corner, it should move at a diagonal to the back left. If it's not moving as well in one direction as it is the other (i.e. backwards is not moving as smoothly as left), then that narrows down which axis is having issues.
Adjust the pulley as needed and make sure the set screw is tight. When you remount the motor make sure to put pressure on it as you screw it back in so that the short belt it is attached to stays taut. If you home the head again, if the fix was successful, the printhead should lock itself in the back left corner and be unable to be moved by hand again until you've either powered off the machine or done something else with the menu to move the head.