Go to your Reddit profile page
Click the username/account name (or “Edit” if available)
Select “Profile” or “User settings”
Find the option to change your username
If username change is not available, create a new account with the desired username
Transfer activity to the new account manually (posts, comments, communities, moderation roles if applicable)
Update your links and any external accounts that point to your old username
If you’re a moderator, remove/assign moderation roles as needed in each subreddit