Your account name is the name that is associated with your account. Your Nickname can be set differently for different servers you are a member of.
- Click on the cog to access your settings
- Click on Edit account.
- You can now change your account name to something of your choice.
- You might want a different Nickname within a server instead of your username. Left-click on your name in the server list- select Change nickname on your profile card.
- Enter your new Nickname and click Save