My sperm in underwear can i reuse underwear or not?

You definitely can, if no better options. For hygiene purposes changing and washing clothes (underwear) before reuse would suffice.

Always hygienic to wash underwear daily- then use it again

Sperm dries up in 3-5 minutes if not in vagina, on surfaces and on clothes or objects it dies on contact after 5 minutes. Your underwear will feel stiff from sperm, but wearable and possibly uncomfortable. Best wash it after reuse, sperm will not damage your underwear.