User passwords expire on a regular basis in Office 365. As an admin, you can make user passwords expire after a certain number of days, or set passwords to never expire.
You must have Office 365 admin permissions to perform these steps.
Sign in with your Office 365 admin account at https://portal.office.com/adminportal/home.
In the Office 365 admin center, choose Settings > Security and privacy, and then click Edit.
If you don't want users to have to change passwords, set Passwords never expire to On.
If you want user passwords to expire, in the first box type how often passwords should expire. Choose a number of days from 14 to 730.
In the second box type when users are notified that their password will expire, and then click Save. Choose a number of days from 1 to 30.
When a user signs in to Office 365 and their password has expired, they will be prompted with a page like the one shown below to create a new password:
Note: This article applies to Office 365 Enterprise, Office 365 Business Essentials, and Office 365 Business Premium. If you're using Office 365 Small Business, check out Change how often passwords expire in Office 365 for Small Business.