Changing your username
Why there is a wait, and what to do if you are locked out.
You can change your username from Settings, but only once every 7 days.
The wait exists because usernames are how people recognise each other, and an account whose name changes daily is hard to trust.
If the field is locked
A padlock means you have changed it within the last week. The date it unlocks is shown beside it.
Usernames are released when accounts are deleted
If a username you want is taken by a deleted account, it becomes available once that account passes its 30-day window.