Please follow the update instruction carefully.

Update instructions from version 7.2 to 7.3 -

If your application has been customized, Then keep your file backup and follow the step-by-step instructions below-
1. Replace the Updater.php file with your existing Updater.php file on your server.
2. Make sure that your server's maximum_upload_size is more than 100MB.
3. Make sure that your server's max_execution_time is minimum 300.
4. Login to the Superadmin panel and go to System Settings. You will get a field of product update. Upload the update_7.3.zip file here.
5. Wait for the update to complete. If you see a white screen, don't be panicked. Clear your browser cache to see correctly.
Window: Ctrl + Shift + R
Mac: Command + Shift + R

If you face any kind of difficulties, let us know through tickets.
That's it.