In a situation where i go to the store and want to ask about the exchange, refund dates, how should i say?

Is it OK to say " when is the exchange due?"

Another question.
The gas in the stove doesnt come out. (by the way, the gas in the stove is right to say?)
How can i say in right way about this?
I am thinking 'gas doesn't come out, gas doesn't(won't) turn up, gas is not on'.
Turn up is right to say?

Help me please!