No, is not possible to downgrade a database. It is not allowed.
What you have you to do is
1. Upgrade your current SQL from 2005 to 2012
2. restore the backup you have on a SQL Server 2012 instance, export all the data and import it on a SQL Server 2005 database.
OR You can create a query to generate sql table structure with data and its constraints
OR You can use functionality called Export Data-Tier Application which generates .bacpac file consisting database schema and data. On destination server, you can use Import Data-Tier Application option which creates and populates new database from pre-created .bacpac file
Right click on database -> Task->Generate Scripts... in Advanced in "Types of data for script" select "Shema and data" And try run this script in your lower version
Hope it helps
Editor, DotNetSpider MVM
Microsoft MVP 2014 [ASP.NET/IIS]