What your looking for in the admin is under settings > general
The easiest method is to have both domains under the same FTP/shell user. Make the change in wp-admin and then rename
newdomain.com -> TEMP, olddomain.com -> newdomain.com, TEMP -> olddomain.com
You will also have additional things to clean up either by searching site manually and editing links or editing the posts table using phpmyadmin, just depends how big the site is and how much experience you have with phpmyadmin as to which is quicker/easier for you.