Do you know what version your .MER is? If it's V5 or greater AND it was compiled allowing you to restore it, then you can restore it back to an editable application using the Application Manager.
If it's V4 or earlier, then it's a lot more involved process that will not give you the entire application back.
I think you mean *.MED file? - Machine Edition Development.
*.APA are APplication Archive files.
You restore *.MED files from either *.APA or *.MER files.
It must be a version 5.0 or newer *.MER file and the "Never allow conversion" option must not have been selected when the *.MER file was originally created.
You need to open Application Manager.
You can open it from Windows "Start/Programs/Rockwell Software/FactoryTalk View/Tools/Application Manager" or from within FactoryTalk View Studio under "Tools" at the top.
Then select "Restore runtime aplication" at the bottom and browse to your *.MER file. Give it a unique name and continue. It should decompile into the "HMI projects" folder and should then be available to select from the "New/Open Machine Edition Application" screen when you start FactoryTalk View Studio.