Hi everyone again,
I couldn’t resist my curiosity and replaced config.php;
‘version’ => ‘10.0.2.1’, to ‘version’ => ‘10.0.0.0’
Then started updater. It worked and it said migration is successful. Disabled maintenance mode and tried to open home page.
I got 500 Error.
From Logs, I saw this:
AH01071: Got error 'PHP message: PHP Fatal error: Interface 'OCP\\Files\\External\\Config\\IBackendProvider' not found in /var/www/vhosts/quantumen-host.com/cs.quantumen-host.com/apps/files_external_ftp/lib/AppInfo/Application.php on line 30\n'
Do you think I can fix this or should I start uploading 100GB again? 
Cheers
Edit:
I have renamed files_external_ftp folder in Apps and later I could see the main page with a screen saying upgrade to nextcloud 12.0.0 with a list of to be updated and deactivated plugins.
Unfortunately, later it said, Exception: Updates between multiple major versions and downgrades are unsupported