Uneven can be a vague term when talking about how your riding lawn mower cuts. Does it scalp the lawn when you turn or cut lower on one side than the other?
Those are two of the most common problems and a third is that the front of the deck cuts more than it should.
Put your riding lawn mower on a flat concrete or gravel surface and visually inspect the deck. Does it lean to the left or right? Is it low in the front and high in the back? Once you determine what the problem is, you can now make adjustments. A tape measure works to adjust the deck from side to side and get it as level as possible. There is a special tool designed for this that measures the height at the blade tips that is available on Amazon or other retailers.
The deck must also be adjusted so the height in the front and rear are correct. A general rule is that there only be 3/4″ difference from front to rear in height. So adjust that accordingly to get the height right.
Are your scalp wheels missing or wearing out? This can cause the deck to scalp the lawn when you make turns. Replace them and the bolts and nuts if they are out of specifications.
These small adjustments can make a world of difference in the look of your yard after you mow, so make them and go test the mower to see if it resolved the problem.