Restoring a backup made in higher versions and restoring it to lower versions is not possible. you need a script to output tables and run them in 2012, you should also consider any 2014 functions that may not work in 2012.
below are the steps
right click on the database -> tasks-> generate scripts

select entire database

in the next step, click "Advanced" and change the properties below

permanently save

you can also use export data as below

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