The downloaded file is a ZIP file of a folder. You need to unzip that somewhere on your PC and then inside of the folder is an Evolution.exe file, which you should be able to double click in order to start it. Let me know if that doesn't work and if so, what happens instead of Evolution launching.
I haven't used the itch.io launcher myself but I suppose that would simplify all of this to just downloading Evolution through the launcher and then clicking "Launch" in there. You could also try that.