Diagnosis
Some wobble at max height is expected, but excessive movement usually means the frame isn't perfectly square or the feet aren't sitting flat.
How to Fix It
Start with the feet. Loosen (don't remove) the bolts that attach the feet to the vertical legs. Let the desk's weight settle the feet flat on your floor, then re-tighten the bolts firmly. Next, check all the crossbar and leg assembly bolts you tightened during setup. Go around and give every single one another quarter-turn to ensure nothing worked loose.
If That Didn't Work
Your floor might be uneven. Get a pack of self-adhesive furniture leveling pads or shims. Place them under the foot that's off the floor. Even a millimeter can make a big difference in stability.
Heads Up
The extended-range 4-leg models are inherently more stable than the 2-leg ones at height. If you have a 2-leg and work with multiple monitors, positioning heavier items closer to the legs/center, not the front edge, helps.