Remove ‘Congratulations! You have successfully installed your Zen Cart™ E-Commerce Solution.’ from Main Page

Go to your cPanel and navigate to /includes/languages/english
Select index.php and click ‘Code Editor’.
Remove Heading Title from Zen Cart - 1
Click ‘Edit’ when prompted.
Remove Heading Title from Zen Cart - 2
In the code, find HEADING_TITLE constant and modify the value as desired.
Remove Heading Title from Zen Cart - 3 (Click to Enlarge)
Click ‘Save Changes’ when done.