Under the enlarger, you can make modest corrections by tilting the easel, and of course you can rotate the easel to straighten the "horizon." For a slight correction, this is often a good trick. Some enlargers also have tilting lens and film stages for even more flexibility in this regard, but ultimately, since the three-dimensional scene has already been converted into a two-dimensional image on film, the most you can do is to square one plane at one distance from the original camera lens, like using rear tilt/swing on a view camera. To correct all planes, you need rise/fall on the camera or a better place to stand.