1- Click on button at the top left corner -> My app Settings - > [ Office365 ]
2- Under the Office365 page -> Security & Privacy -> [Password]
3-Under Password pages:
You must also change the password at your Microsoft Outlook application in your computer and mobile/tablet devices after changing your password.
Password policy
When you create a new password, use the following guidelines:
What to do if you forget your password?
If you forget your password, you must request a password reset from an Office 365 administrator. Your administrator can provide you with a new temporary password to use the next time you sign in.