To set an expiration date for passwords:

1. On the left panel of your admin dashboard, head to People > Settings.

2. Click on the App Settings tab.

3. On the left of the section, click Password Settings.

4. Toggle Activate Employee's Password Expiry? switch ON (green).

5. Select number of days to password expiry. After this number of days, employees’ passwords will

    expire and they will be required to change it.

Read more: 

How to activate a rule for unique passwords