If you would like to change it back so that your login screen reads Zorin OS 6, follow the steps below.
1. Obtain the original Zorin OS 6 logo. If you have the original live DVD you used to install Zorin, boot the live DVD. Once you have Zorin running, open Nautilus (file manager) and navigate to the /usr/share/unity-greeter folder. Look for logo.png. Copy this to a flash drive. Maybe you have another computer with Zorin installed on it, you can copy the needed file this way too.
2. Go back to the computer that has the incorrect logo and insert the flash drive containing the correct logo.png file.
3. Open terminal and type sudo nautilus.
4. Navigate to the /usr/share/unity-greeter folder and copy the logo.png file from the flash drive to the hard drive. Make sure logo.png has the following file permissions (see screenshot below). Right click file name > Properties > Permissions.
5. Reboot computer and now the Zorin OS 6 logo is back!
Ubuntu Tweak is another way of changing the logo in the login screen. In addition, you can also change the login screen background. (see fig. 1)
|fig. 1 - Use Ubuntu Tweak to change logo and background|
If you do not see this option in Ubuntu Tweak, you may have an older version and may need to update it.