I can’t find out how to edit the titles of my activities but I see others who have. If I’m using my PC (windows 10) I can’t see any edit option. But if I hover the cursor just below the flag in my feed list there is a hidden edit button which does nothing. On my android app, again there is no edit option either.
You can name them in the ‘Save and Exit’ screen in game or on your mobile device. While it does say “Zwift - <DATE>” you can highlight it and change it before hitting ‘Save.’ We do not have a way of editing them after upload currently.