The way to fix uneven abs depends on the reason why the development is staggered. If the cause is genetic, there is little to nothing that can be done. However, if it is due to the effect of uneven exercise, focusing on exercising the portion of muscle that is smaller develops it. Additionally, uneven abs can result from fat distribution, so dietary changes can also help.
For some people, uneven abs result from simple genetics, or the way they are built. According to Your Fitness Corner, this may make it appear as if the two sides of the abdominal muscles don't line up perfectly. In these cases, there isn't anything to do. However, according to the same source, this is something that affects even the most successful body-builders, and it doesn't stop them from winning tournaments. If individuals' abs are underdeveloped due to exercise inefficiency, they can start by focusing on work-outs that specifically build the sections of muscle that concern them. For example, reverse crunches and hanging knee raises target the upper abdominal sheet, whereas decline crunches target the lower sheet. There are also many exercises that target the obliques, or side abs. Due to evolutionary factors, the human body is naturally predisposed to storing more fat in the lower abs, which explains why that particular fat is often the hardest to lose. If uneven abs are due to this stubborn lower abdominal fat, then changing the individual's diet to have a lower body-fat ratio is necessary. In time, the lower abs flatten to match the upper portion.