This is ridiculous that you still haven't provided this option.
Updated on 2020-01-24
Thanks for sharing your idea here in the Community!
Keep in mind that this has already been suggested here: https://community.spotify.com/t5/Live-Ideas/Account-Change-Username/idi-p/703799
We'd recommend heading over to that idea and adding your +VOTE and feedback there.
More information about how the Spotify Idea Exchange works can be found here.