If your password has already expired, you won't be able to log into VPN in order to change your password the "normal" way (via Enterprise Connect). Here's instructions to change it. Make sure to follow both steps, not just Step 1.
Step 1: You'll need to go to pm.interpublic.com, log in with your full email address (not just first.last) and expired password, and follow the prompts to change your password there.
Once that is completed, your computer login password will still be your old expired password, and we don't want that, as it is annoying and could cause a lot of issues down the line. So....
Step 2: Follow the instructions at the following link in order to get your computer login updated to your new password: Re-sync Mac login password and Network/Email password