I think you can only crop from the website, but not on the phone apps (at least not Android. Dunno about iPhone). From PC/Mac, click the wrench icon, then Crop and go from there...use the sliders to change the beginning and/or endpoints.
There may be a 3rd party app that can crop files, maybe GPS Prune (I haven't tried it though)... see DC Rainmaker'Rainmaker' list of useful apps here: https://www.dcrainmaker.com/tools
You could visit the website on your phone. Should work, but might not be ideal.
So the website is worthless on an Android phone, I can get to the trim function, but can't do anything.
I tried opening Strava directly in Chrome. It worked eventually