In order to force a user to change their password the next time they login, under the settings wheel on the top right navigation bar, choose "Password Settings".
Next under "Password Settings" click the option "Enforce Password Expiration" and select the amout of days for the expiration. Then, make sure to save at the bottom.
Next, click "Users" on the left side bar navigation menu and choose the user you would like to edit using the pencil icon next to their name. Once in their profile, check the box that says "Force user to change password on next login" and make sure to save at the bottom of the page.