This question already has an answer here:
How to get rid of the name or you can just see my name is user2060755. So how to set my original name?
You need to navigate to your profile. Then you need to edit your profile (edit button in the top middle right). You can change your username there, and then save your changed name. You can only do this once every 30 days.
In fact, I made a typo making my name and have to wait 10 more days to fix it.