I have young Avocado plant in the pot with leaves turned dry brown on the edges. It seems like the soil is infested resulting with small dark flies hatching from it. How to cure it?

Repot it immediately into fresh "clean" soil. Wash the roots with tepid water that way you can wash away any of the old infected soil as you don't want to transfer any larvae of the gnats into your new pot. You need to let this plant dry between waterings. Keep it warm and in the shade. They like bright but indirect sunlight. Fungus gnats, the dark flies that you see around, come from wet soggy soil..don't over water..Let the soil dry out and then water your plants. Your avocado plant should recover fine:)