The command line is documented in the WSUS Operations Guide on page 65.
WSUS operation guide - Microsoft Download Center

Here is the process you would need to follow:
1. Create a folder called "content" on the new partition (e.g. E:\content)
2. Then use the following command: %drive%\Program Files\Update Services\Tools>
    wsusutil movecontent E:\content C:\movecontent.log
This command will move the content folder from its current location to E:\content and create a log file called movecontent.log on the root of your C:\ drive.