Hi Kabeer,

The data migration is handled by Odoo when you have an enterprise license. You can simply upload the database to http://upgrade.odoo.com after which Odoo will handle and migrate the database.
A few weeks after you've sent in the test migration you will get a response from Odoo that the database is ready for testing. Restore it locally and make sure everything is fine on your upgraded database. If it is not the case you can notify Odoo, otherwise you can continue.

When you've passed this phase the next time you'll upload your database again it will be for migrating it for real this time. Odoo has mapped all changes that need to be done to your database and will do the upgrade automatically the second time, meaning you'll get the database back within minutes (usually less than an hour).
Make sure that you've migrated all your custom development from V11 to V12 before you do this part of the step.

Then, finally, when all the custom development is migrated and your database is migrated by Odoo you can restore the dump on your new odoo v12 installation. If you did everything right it'll work out of the box but usually it still needs some last minute work though. Preparation is key for a migration.

Tip: Read through all the questions and answers on the Odoo upgrade website, it contains a lot of handy information about the process.

Regards,
Yenthe