Once you have updated to Mabui 77 go to "Studio". Template apps that you have published will be highlighted in bright blue to indicate that they need to be migrated. You will also see a message asking you to migrate your template apps.
Just tap on a highlighted template app, and you will be taken directly to the publishing screen. Use the "Start pre-migration checks" button followed by "Go to publishing" then "Migrate app". You should then see the message "Thank you - this app has now been successfully migrated to the new platform." and the blue highlight will be removed from studio. You can then update your template app from studio as before.
New users will require Mabui 77 in order to use the migrated template.