This article is for cPanel Accounts. Determine where your account is with this guide.
You will want to update all contact information and billing information to the new person. This may include the domain registration's WHOIS information, the cPanel contact information, and your WestHost account billing data.
This can almost entirely be handled through https://cp.westhost.com/ under Contact Information and Billing >> Credit Card.
To update the WHOIS information for a domain registered through WestHost, you will need to log into it directly at http://access.enom.com/ with that domain's unique password.
You and the new owner are liable for updating all information -- WestHost may not be able to refund any charges that are made on old billing information if it was not updated correctly during the transfer of ownership.