The .prg is on the watch in \garmin\apps, and you'll have to try and figure out which it is as the app store gives it an 8 character name.
if it uses app settings (from GCM or GE) you won't be able to use that by copying it over. Also the display on the f5x is different than the f3 (240x240 vs 218x218) and the fonts are different, so how it looks on the f5x could be an issue.
In this post I show the same watchface (in the CIQ simulator) for the f3 and f5, can you can see the font differences, for example, and this is one where I changed the code for the change in fonts. The unmodified .prg would have looked bad on the f5.
https://forums.garmin.com/showthread.php?370152-Fonts-and-altitude-issues&p=948257#post948257
The .prg is on the watch in \garmin\apps, and you'll have to try and figure out which it is as the app store gives it an 8 character name.
.....
"Real" APPS names (.prg files) you should search for string ".mc".
Actually not. You'll find the names of the source files involved (an .mc is the source file), which MAY relate to the program name (they don't have to). You could find a strong hint though... (myAppView.mc and myAppApp.mc could be used to build "XYZthing" for example )
Toward the end of the .prg is the String resource that's shown on a watch for the name an app though, but it's mixed in with the other strings used. You could search for that to see if it's the correct .prg