Your username is your public name on the site. It is shown with an @ in front, on your profile page (/user/@name), next to your comments, reviews and verdicts, and in lists of users. Other people find and mention you by it.
A username is 5 to 20 characters long and uses only Latin letters, digits and underscores. It must start and end with a letter or a digit, and two underscores cannot follow each other. You can type it with or without the leading @; the site adds the @ itself.
Some names are not available. Usernames starting with bot_ are reserved for bot accounts, and names that match the site blocklist are refused. If a name is not accepted, the settings page shows the reason and keeps your old username.
To change your username, open Settings, Profile, type the new name and press "Save username". The change applies at once and your profile address changes with it, so links to your old /user/@name stop pointing to you. Use the info icon next to the username on the settings page to open this page.