There is no way to achieve this - you can never go back in time with SQL Server.
That is: you cannot with any method or trick or hack or workaround attach or restore a newer database (file) to an older SQL Server version.
No way. Doesn't work. Don't even try it. Nope.
You'll need to find a way to synchronize the two databases - both their structure (tables, views, stored procedures etc.) and their data (contents of the tables) - manually, e.g. by using T-SQL scripts to update one database from the other, a tool to help you with that, or whatever means you want to use.
But you cannot in any way, shape or form detach/attach or backup/restore newer SQL Server database files onto an older version.