It’s a matter of balance.
In chemistry, the word for balance is equilibrium. If something is in equilibrium, the amounts of things in it stay unchanged.
A snowflake is an equilibrium between solid water and liquid water. The bulk of the snowflake is solid, but at the surface there is a layer of liquid water, and some of that surface water escapes to become the gas water vapor. Molecules of water can also escape directly from the solid ice into the air to become water vapor directly.
What keeps the snowflake intact is equilibrium. For each water molecule that leaves the ice to become water or water vapor, there is a molecule of water or water vapor that crystallizes onto the surface of the ice.
If we disturb the balance, we can either make more atoms solidify, forming more ice, or we can make more atoms liquefy, forming more water. We change the balance (or shift the equilibrium) by changing the temperature, or changing the pressure.
Heat is the motion of molecules. If we add heat to something, we raise the temperature, which is the speed of the molecules. In the ice, the molecules are bouncing around against one another, vibrating back and forth. If we increase the speed, they can jostle loose and become liquid water.
If we increase the pressure, the molecules on the outside of the ice are pushed back towards the ice, where they have a better chance of sticking to the other molecules there. So increasing the pressure makes more ice, and decreasing the pressure makes more water or water vapor.