If you would like to change your Trainer nickname, you can change it within the Settings menu. You can only change your nickname a limited number of times, so choose wisely. Keep in mind that your nickname will be displayed to other Trainers.
To change your nickname:
From the Map View, tap the Main Menu
At the top right, tap Settings
Tap Account
At the top of the Account page, you will see your current Trainer Nickname, tap the pencil icon to change your Nickname. Note: If you cannot see the pencil icon, you have already redeemed all available nickname changes.
Read the disclosure about how often you can change your Trainer Nickname and tap either Yes to move forward or No to go back.
If you have tapped Yes, you can enter your new Trainer Nickname and tap Yes to complete the Trainer Nickname
Note that if you have chosen a Trainer Nickname that is already in use by another Trainer, a number of variations on the Nickname you’ve entered will be shown for you to choose from.