The quick version
The process for installation over an existing Gallery+ is to deactivate the existing one, install and activate the new Gallery+ using the WordPress plugin installer, and then work through all existing Gallery+ blocks ensuring all their settings are correct.
1. In your WordPress, under the Plugins section, find and deactivate your existing Gallery+ block plugin. If you do not deactivate the old one first, you will get a fatal error message.
2. Still in the Plugins section, select the ‘Add New’ plugin option and upload the Gallery+ ZIP file.
3. Activate it when prompted
That’s it! You can now start exploring Gallery+’s cool features.