You can modify your display name at any time by opening Spotify, going to your profile, and then tapping or clicking on it. However, keep in mind that your Spotify username is fixed and cannot be altered, so only the display name will change.
You can’t change your actual Spotify username, only your display name. Just go to Your Profile - Edit - Display Name, update it, and it should sync after restarting the app.
Spotify doesn’t let you change your actual username, but you can change your display name.
Go to Spotify app → Settings → View Profile → Edit Profile → Display Name → Save.
Your new name will show on your profile and playlists.