User passwords expire on a regular basis in Office 365. As an admin, you can make the user’s password expire after a certain number of days. You can also change the number of days, before users are notified of password expiration.

1. Go to “https://login.microsoftonline.com/ and login with your credentials.
2. Click on “Admin” in the top – right and then select “Office 365” from the drop down.
3. Click on ”Service Settings” and then click on “Edit” under the “Password expiration policy. 
   
4. Specify the number of days as the password life, after which it expires and user is prompted to change his/her password. You can also set the number of days, before which user will receive notification about password expiration. Click on “Save” to close the dialog box.
5. Your password expiration policy has changed successfully and the new values are reflected under the “Password expiration policy” group.