I have an upstairs shower/tub unit that we do not use very often that has a septic smell coming from it,what can I do?

If it's used infrequently it may be that the water in the trap which prevents sewer odors from coming out of the drain has evaporated. Run some water in the tub and that should fix it.

Agree with ebaines. To keep the water from evaporating, fill the trap with water and pour a little cooking oil on top, a tablespoon or two should work.