Question

for any substance, why does the temperature remains constant?​

Answer 1

The temperature remain constant due to the latent heat

Secondly I think your question is in complete...

(where is change of state??)

if you do mean that...then this your answer

The temperature remain constant because the extra supplied to it is used in overcoming the force of attraction between the molecules.

So there is a no rise in temperature

EXAMPLE:

if ice changes to water, it's temperature remain same until all the ice has melt... the heat supplied to it is used to change the state

if water changes to steam, it's temperature remain constant until all the water have evaporated..