You know your username, but forgot the password and need to reset it
If your admin has given you permission to reset your password go to, passwordreset.microsoftonline.com/. Contact your admin if you don't have permission to change your password.
You know your password, but you want to change it
You can only do this if your admin has given you permission to update your password in Office 365.
Go to www.office.com/signin and sign in with your work or school account username and existing password.
Go to Settings > Password.
Make your changes and click Save.