You cannot change your Reddit username after creating the account
You can change your display name on your profile if you want a different public name
To use a different username, create a new Reddit account with the username you want
If you signed up with Apple or Google, your account may have a randomly generated username that can only be changed once during the initial setup period, if available
Check your account settings or profile for any option to set or change your username during account creation
If you want to keep your posts or comments, you will need to use your existing account because usernames cannot be transferred