What is the ruling on sweat and moisture, accompanied by an odor tending towards feces, in the anal area in the morning, and is it obligatory to change the underwear because of it?

If sweat accompanied by moisture is present after an adequate istinja’ (cleaning oneself after relieving oneself), it is excused and considered pure (tahir), and there is no need to wash it or change clothes. As for an odor, if it is not associated with a physical substance, such as the lingering effect of passing wind, it is pure and not considered impure (najis).