To change your name in Legendary just follow the steps listed below:
1. Tap the Gems icon located to the left of your Gems balance.
2. Once inside the Gem store menu, select the Learn More button located under Name Change.
3. Once inside the Name Change menu, you can purchase a Name Change for 300 Gems. (Changing ones name is associated with a cost to prevent players from readily abusing name changing, while still making this easily accessible to everybody.)
4. After you have purchased your Name Change, maneuver over to the settings menu.
5. Tap Change Nickname from within the settings menu
6. Enter in your new Nickname, and tap the Change Nickname icon. You're all set!