To migrate a database from one SQL version to an older SQL Version, the usual options like backup-restore and detach-attach are not supported.
One of the options available to migrate a database with an SQL version to an older SQL version is by using the Import and Export Data Wizard.
You can find below the steps you will have to execute in order to migrate a database using this tool and other things you have to consider when doing a migration:
Steps to use Import and Export Data Wizard:
use <name of the database >
execute sp_MSforeachtable 'select count(*) from ?'
In order to apply with success the steps described above, the one who executes the steps must know the structure of the database and also all other options configured for this database. Also, the same person must be aware of any job, alert or other configurations done on the database in question.
The option presented above it is not the only one that can be used to migrate a database. You can find all the options available, in the SQL Server Books Online and you can choose the one that best fits to your needs.