Please change the recipient for the documents sent to Company C from Mr/s. A to Mr/s. B.
Regarding the documents sent to Company C, change the recipient from Mr/s. A to Mr/s B.
「C社へ送る書類に関して、Aさん宛からBさん宛に変更してね」は英語で「
Please change the recipient for the documents sent to Company C from Mr/s. A to Mr/s. B.」という表現が良いと思います。直訳に「Regarding the documents sent to Company C, change the recipient from Mr/s. A to Mr/s B.」です。どちらでも良いと思います。