Most contact information can be changed directly in your account by logging in and going to Domains > View Domain > Contact Information with two exceptions which require manual paperwork for security reasons.
Update of the registrant contact, the registrant contact is the primary contact for the domain name and is where passwords like EPP codes are sent on request. If you need to update your contact details please download and fill out this form. Please note the form will allow you to manually update your details but will not change the legal owner of your domain name.
Change of legal owner - this is who the domain name is licenced to. For a change of legal owner & domain sales, see here for details.