上着 and 下着: outer garments and underwear
Read clothing names by how they are worn and follow action order.
An outer garment
上着 (うわぎ) means an outer garment such as a jacket. 「寒いので上着を着ます」 says the speaker will put one on because it is cold.
「上着を脱いで椅子に掛けました」 means taking off the outer garment and hanging it on a chair. Check the order of 脱ぐ and 掛ける.
Underwear
下着 (したぎ) means underwear. 「旅行のために下着を用意しました」 says underwear was prepared for a trip.
「下着を洗濯しました」 means washing underwear. The character 下 does not make it a word for all lower-body clothing such as trousers.
Part versus whole
「上着だけ替えました」 says only the outer garment was changed, not every item of clothing.
「下着も一緒に入れてください」 requests including underwear as well. も adds it to the items already under discussion.
Does 下着 mean all lower-body clothing such as trousers or skirts?
Show the answer and why
No. It means underwear; do not infer the meaning from 下 alone.