The answer is yes, you can change your account name or nickname anytime that you want. Unlike with the other games that you get to be stuck with the name that you have used the first time.
Anyway you will be able to change your name by following the instructions below;
Changing Account Name or Nickname
1. Tap the upper left of the screen, you will be able to see the Menu icon.
2. After opening the Menu, go to Options then proceed to Settings.
3. After opening Settings, you should be able to see this;
4. Tap the name area to be able to modify the current name then hit the Change button to finalize the changes.
5. After finishing, you should see immediately the new name that you have chosen.
"Your name is a representation of yourself. Keep it good, witty, fun and clean - gumi Team"