Refer to the following for password change procedures.
Change password (Web browser)
https://teachme.jp/8/manuals/4554233
Change password (Mobile App)
https://teachme.jp/8/manuals/3204731
Related: When changing the login password, the message "Please use a more complex password" appears. What should be done?
https://help.teachme.jp/hc/en-us/articles/900005510526