changing one's clothes; a formal term used for changing attire (often seen in compounds like 更衣室 — changing room).
試験の前に更衣して制服に着替えた。
Before the exam I changed into my uniform.
化粧室で更衣を済ませてから式場へ向かった。
After changing in the restroom, I headed to the venue.
archaic/historical sense: a court lady or attendant in the imperial court; used in historical or literary contexts.
平安時代の物語には、更衣が登場する場面がある。
Heian-period tales sometimes include scenes featuring court ladies (kōi).