How convert database from SQL Server 2012 TO SQL Server 2005 I have SQL Server 2012 database which I need to use. But I can't install SQL Server 2012 on my PC. 

I use SQL Server 2005 only.

How can I change the database SQL Server 2012 To SQL Server 2005 OR 2008 

Or help with installing SQL Server 2012
If you can't install SQL Server on your PC the only choice you have (if the database is already 2012) is to script out the databases objects, change anything that isn't compatible with 2005, and then run the scripts for your 2005 instance.

Because there are many changes between 2005 and 2012, datatypes among other database objects may not align properly. This means the application(s) being used with the database if put into 2005 from 2012 may not function correctly or at all.

Your best option is to get a waiver to install 2012 developer edition or some other test system for your 2012 needs.