Mission impossible or have I overseen something?
There is a delete account button on your profile screen. I assume it works. Click on your profile icon in the upper right corner and select the tiny gear. At the very bottom of the page is the delete account button. I don't see any way to change your username otherwise.
Thank you for your quick reply! I have done this for my wiki account but for my forum account there is no such at the location you described. Please take a look at my screenshot to see if we are talking about the same location.
That's strange, because mine does...
Indeed. Perhaps some kind of "no, not you...don't leave" ;- ) Will recheck some days later and / or another browser. Thank you for providing the evidence that I am not too dumb to find a big red button ;- )
Essentially is comes down to - you have made more than 1 post. This is to preserve the integrity of the community. If you request it, I can remove your posts, suspend your account and/or change your username.
Some additional info at:
https://meta.discourse.org/t/how-to-delete-user-account/16908
Thank you thess for helping!
@thess
I'm having a similar problem as described.
I wanted to rename my account "adridolf" to "adrianschmutzler". Since rename is not possible, I created a new account "adrianschmutzler" but cannot delete the old one ("adridolf").
As you suggested this for the earlier question, it would be nice if you could delete "adrianschmutzler" again and then just rename "adridolf" to "adrianschmutzler", so I keep the old history.
I will post with my new account here in a minute, so you know it's mine, too. 
Done, you are now adrianschmutzler.
Thanks, that was really quick!