This is a frequent question, and one that has an answer that you may not like: attempting to shrink leather permanently will ruin it.
Heat will shrink leather, but when it shrinks, it causes the cellulose fiber in the leather to lose its elasticity. That means that the area of the leather exposed to heat will become brittle. It will eventually crack, break, and form a hole. Often leather that is exposed to heat shrinks unevenly, meaning that it may shrink too much in one place, and too little in another, forming an overall misshapen boot. Don't do it! Do NOT expose boots to heat sources to try to shrink them!
It is an “old wives tale” that you can put boots in an oven, or pass a flame from a candle or a lighter over the leather to get it to shrink. Yes, it will shrink, but in a bad way, as described above.
If you have boots that are too large in the foot, try getting good quality gel insoles to take up room. Wear thick, quality socks, made of fibres including wool, Daycron, Nylon, and cotton.
If you have boots that have a large calf circumference, you may consider wearing jeans or leather inside of them, which will take up extra room. Also consider thick wool boot socks, as well.
Summary: It is NOT possible to shrink a leather boot permanently without causing damage. Your alternatives are to take up room with insoles and socks, or dispose of the boots by selling, donating, or giving them away.