Use the application manager (in the FactoryTalk View Studio Tools folder), and select "restore runtime application". That'll restore it back into FTView Studio, and it will appear in your list to select when you start FTView Studio.
As jkerekes is alluding to though, it's not certain that'll just straight up work for you. Some older versions of .mer files don't allow conversion. Even the newer ones, whoever created the runtime file can select "always allow conversion", "allow conversion with password" or "never allow conversion". If you don't have a copy of the program beyond the one you uploaded, I'm going to guess that you also don't have the password, so unless they selected "always allow" you could be at a dead end anyway.
Pro tip: if it asks you for a password, try leaving it blank and pressing "OK" or enter - by default, the software selects the password option, so if you're not paying attention and just click through you'll end up with a password protected runtime file, but the password is blank. You might just get lucky.
The other thing to be aware of is that generally, when you restore the application, all of the user data (usernames and passwords) is scrambled. So make sure to check that out, of you intend to edit and re-download the application you'll need to get a hold of all the usernames and passwords ahead of time, and make sure you can work out which login had which access codes.
Good luck!