Hi
If you renamed the template , it will not longer appear as JA Product , you can try to change the name to back to JA Megastore from template details.xml file . Or install the latest version of template and assign it to your pages .
If you done a heavy custom work in template files and unable to do above process , you can update the template files manually via JA Extension manager > Templates > JA Megastore > Check for updates > Compare tool . Compare files and manually update .
Do not forget to take full backup of site before updating the template .
Check upgrade steps Here