To change the orientation (rotate) of a video already registered in Teachme Biz, you can edit it in the web browser version.
Rotate video (Web Browser)
https://teachme.jp/8/manuals/8081285
※Videos with added markings or subtitles cannot be rotated.
If you have already added markings or subtitles, please delete them all before changing the orientation.
Related:
Add captions, symbols, or frames to video (Web Browser)
https://teachme.jp/8/manuals/14366698
Add subtitles to video (Web Browser)
https://teachme.jp/8/manuals/15272626