This can be accomplished via PowerShell. Note that this does not change the policy applied to new users, but instead changes existing user settings.
Pre-requisite: Connect to your Microsoft Online account using the PowerShell profile script provided on this site under Tips & Tricks, or load the MS Online cmdlets.
Run PowerShell as Administrator, then issue the following command:
get-msoluser -all | set-msoluser -passwordneverexpires $true
This command will get all users and set their PasswordNeverExpires property to true. Note that if you have many users, it will take a while to run.
To set a specific user's password expiration policy, run the following (replacing the sample user name with the actual user name):
get-msoluser -SearchString "Jack Harkness" | set-msoluser -passwordneverexpires $true
If you run into any issues, please create a helpdesk ticket!