Okay, so these screenshots should help I think? At least on Android.
Open your sound observation and follow the red circles on the first two screenshots. On the third, you’ll just need to input the time/location/ID data- it won’t export metadata automatically from merlin, unfortunately. Usually what I do is just upload the bird song when I record it (if I have wifi) so I can just the automatic GPS location.
I don’t believe Merlin will let you edit sound files in the app, unfortunately; if you want to shorten them I think you need to use external tools.