Login to your Account Portal and select a subaccount by clicking the "View Subaccount" button next to the subaccount listed on your Subaccount Page.
Then navigate to the Numbers tab and click the phone number(s) you wish to delete. For each phone number click the "Release Number" link in the lower right and confirm your action.
Please note that if you close your subaccount, all of the numbers in that subaccount will be deleted automatically.
You can also use the REST API to delete phone numbers.
For more information on deleting phone numbers please see Cancel or release a Twilio number.