Boots are a lot like people who hold grudges: unforgiving. With no laces to loosen, boots that are too tight will generally stay too tight – unless you coax each one into fitting the shape of your specific foot. That's especially important for anyone with high arches. That arch pushes the foot up into the instep (the part of the boot covering the top of the foot), so it's the instep to focus on when stretching boots.
Grab Shoe-Stretch Spray
Shoe-stretch spray is made for exactly this purpose. It loosens the fibers of leather or suede boots, making the material more pliable. This type of spray tends to be less effective on synthetic materials, but may still make a difference. It's affordable so it's generally worth trying. Find it online, in any stores that sell shoes and even in grocery and drugstores.
The trickiest part about using stretch spray for high arches is getting the liquid on the inside of the instep. Avoid spraying it on the outside of the boots, if possible; these sprays shouldn't cause discoloration, but it's not worth the risk. Holding a boot upside down, snake your hand up inside it and spritz the spray onto the entire area that covers the foot's arch. Put the boots on immediately and wear them for a few hours. The softened material should give a little and mold to the foot.
Use a Shoe Stretcher
If the insteps are really tight, wearing freshly-sprayed boots may not do the trick. Take the technique a step further with a boot stretcher. This device has a two-part wooden or metal piece that mimics the shape of the foot, attached to a long handle. As the handle is turned, the stretcher expands and forces the material to expand too. Make sure the one you choose is marketed as an instep stretcher or vamp raiser, as opposed to a stretcher designed to widen shoes.
Again, start by spritzing the inside of the boot with stretching spray, then insert the boot stretcher and turn the handle until it can't be turned any further. Leave the boots overnight and remove the stretcher in the morning.
Try Less Conventional Techniques
Using a boot stretcher is usually effective, but buying such a device might not be in the budget. In that case, it's worth trying a home remedy first. One option? Fill heavy-duty food storage bags almost completely full with water, tuck them snugly into the foot of each boot and put boots in the freezer overnight. Water expands as it freezes, stretching the boot. Let the ice thaw before removing the bags.
Putting on thick socks, wearing the boots and aiming a blow dryer at the tight area might work too. Use a high setting and flex your arches to force the boot material up. If even that doesn't fix the issue, take the boots to a shoe repair shop. These shops have a range of tools that can be used to stretch insteps.