Log in to the account or service where you need to change your name/username. This could be social media profiles, email, gaming services, messaging apps, etc.
Look for the "Profile", Settings" or "Account" section by clicking on your icon/avatar in the top menu bar.
Once in your profile or account settings page, search for an option called "Edit profile" or something similar.
You should see fields to edit your display name, username, or both. Some platforms separate them while others combine them.
Input your new preferred name or username. Make sure it follows any character or length limits for that service.