The Magento Base URL is the URL used to access Store pages. It can be set on a per Store basis, so every Store could have its own Base URL.
Before starting the changes please make sure that the domain you wish to use as Base URL is already resolved to the IP-address of your Store (so either domain’s DNS servers or IP-address were set to Magenting’s servers). To verify that DNS changes are done, you can just open the domain name in browser. If DNS is set correctly it will redirect you to current Base URL (in .magenting.com subdomain).
To change the Magento Base URL you need to switch to Magento Admin Area and modify the Base URL at System > Configuration > Web > Unsecure > Base URL and System > Configuration > Web > Secure > Base URL.