Under which situations can an amorphous substance change to crystalline form?

Hint: Crystalline materials are the substance that are firm, rigid and incompressible and hold a definite and fixed shape. This is due to the arrangement of the particles in a crystalline solid that are arranged in a repeating pattern of a three-dimensional network, also known as a Crystal Lattice structure.
In contrast,Amorphous materials are the substances that have no detectable crystal structure i.e. they are rigid structures but they lack a well-defined geometric shape and the particles of matter do not form the three-dimensional crystal lattice structure that we see in solids. The most common example of an amorphous solid is Glass.

Complete step by step answer:
In order to convert it to its crystalline form, amorphous substances must be kept on heating at a temperature just below its melting point and then quickly be allowed to cool down rapidly.
The process of heating at a temperature just below its melting point for long periods of time would initiate solidification of the melt through very slow cooling down so that the molecules or macromolecules of the material would get sufficient time and opportunity to gradually re-arrange themselves into a highly ordered crystalline lattice structure.

Note: One must keep in mind that Crystalline materials have sharp, well-defined melting points whereas amorphous substances, on the contrary, do not have the same as they melt over a wide range of temperatures .