Harness boots look great, and, if you invest in a quality pair, they can last you for years. The only problem with a good, sturdy pair of harness boots is that they might take you a long time to break in. If your boots are leather, eventually they should relax a bit and stretch out to fit your feet perfectly. If you bought synthetic boots, though, don't expect to be able to break them in.
Put your harness boots on and check where they are pinching you. If you have a high instep, they are probably pinching under the harness. If your foot is wide, they might be pinching at the sides. In both of these cases, you probably can break them in. If the shoes are hurting your toes, however, they may be too small for you. If the soles of the boots are not long enough for your feet, then the boots don't fit you. Exchange them for a bigger pair.
Wear your harness boots for a few hours every day. The best way to break in a pair of boots is over time. Slowly, the leather should stretch out to fit your feet comfortably. Don't wear the shoes to work or somewhere where you'll be on your feet all day because you could end up with blisters and sore feet. Instead, begin by wearing them around your house. This way you can start to break them in, but will still be able to return them if you decide they don't fit.
Wear your boots on short errands outside of your house once you decide for sure that you're keeping them. You'll be able to speed up the break in process without hurting your feet.
Take your boots to a cobbler or shoe repair shop and have them professionally stretched. The harness on harness boots can be difficult to break in because it isn't actually in contact with your foot. You'll need to stretch at least two layers of leather. A cobbler can stretch the shoes for you without damaging them.
Soak your boots completely through in warm water in your bathtub. Put the wet boots on your feet and wear them all over until they are completely dry. This method works on most boots. Don't try this if the harness boots are made of soft leather, though, or you'll damage them.