If the map you've downloaded has been properly unlocked and it's an older device then the file should work without anything further being done. Best to say which device model though.

JCV files aren't locked like map IMG files. We also need more info to properly advise you however. There are different JCV files, some are quite device specific others are broadly compatible ... e.g.: Some JCV files aren't interchangeable at all, however some split-screen and full-screen JCV files of the same 'type' are interchangeable but perhaps at the expense of picture quality. In short, it's 'horses for courses' and best to disclose completely the device you're updating and which JCV file you want to use (if you already have it).