Magento Base URL is the URL used to access Store pages. You can set it on a per Store basis.
Before starting the changes please make sure that the domain you wish to use as a Base URL is already resolving 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 completed, you can open the domain name in your browser.
To change Magento Base URL you need to switch to Magento Admin Area and modify the Base URL option at System > Configuration > Web > Unsecure > Base URL and System > Configuration > Web > Secure > Base URL.