I don't have a terminal in front of me to check, but I think it's in the configuration menus under Terminal Settings, maybe "FactoryTalk View ME Station" settings.
In general, a newer PV+ can run an older *.MER runtime.
So if you have access to the FactoryTalk View Studio software, just create a Runtime with the appropriate firmware to match the terminal, or older.
If you don't have access to the FactoryTalk View Studio software, you'll have to try to figure out which revision it was created with, and upgrade the terminal to match. This opens up the can of worms about the difference between the PV+ and the PV+6 hardware.
There was a thread a couple of weeks ago about how to determine the "version created with" for a FactorTalk View ME runtime file. I didn't follow it but it might be helpful to you.
So the basic task is to figure out exactly what you've got: look at the exact part number and the sticker on the terminal to figure out which version it was shipped with, at least.