Usernames cannot be combined or merged, however you can reconfigure the Mozy software to reassign the computer to another username. This will add the machine to the other username as a new machine, and it will require a re-upload of the data it has been backing up. 

To reconfigure the backup software:

Windows

1. Hold the Shift key while right-clicking the Mozy icon in the System Tray by the time and date on your machine.
2. Select Launch Setup Wizard from the pop-up menu.
3. Enter the username and password for the user you are moving it to.
4. Complete the set up, and make sure to re-select your files for backup so they can be backed up again.

Mac

1. Hold the Option Key and click on the MozyPro icon in the upper menu bar to select Uninstall.
2. When the window comes up, deselect Keep configuration and log files.
3. Click Uninstall.

Then reinstall the latest version and make sure to put the username and password you will now be using. Reselect your files for backup before finishing.