A nickname is your in game name("IGN" for short) which will display in various location in the game, make other players recognize you easier. Your current IGN will display at the top left of the screen.
How to Change Nickname
Tap the mouse like icon with "Other" below it, and the menu like the picture below will show up:
Tap the icon with "Nickname" below it, and it will bring you into the menu like the picture below:
Enter your nickname, you can use numbers and alphabets.
Note : Respect the other players by avoid using swear words as well as dirty words.