Those cookies are issued based on the current domain on which the site is running. The domain is automatically picked up from the IIS settings for the site. If the Sitefinity site have multisite enabled then each site has its own domain configured via editing the site properties
Let me know what is the need of updating the cookie domain, does an incorrect domain gets picked up? If this is the case this must be due to the server setup on which the site is running, a reverse proxy or other modification that makes the site run on one domain while the server hosting it runs on another. The cookie may pickup the domain from the server and not the actual site url and to correct this the setup of servers must be altered. Let me know details if this is the case.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items