Here's a simple guide for you to change a base domain name or site url for WordPress and MPMU.

1>  Go to the below link to download a tool which named Search-Replace-DB to your local.
      https://github.com/interconnectit/Search-Replace-DB



2> Upload the tool script folder to your site root folder.( eg. the same folder as wp-config.php or wp-content.php)

3>  Browse to the script folder URL in your web browser. 


4>Fill the fields as needed.

5> Click the "live run" button to start searching and replacing.

6> After you replaced,  please delete the script folder in your account to secure your account.

7> Edit wp-config.php, replace old domain with new domain if it has site url settings.