You can update your display name in two ways:
From Your Profile
1. Log into the g+ app and tap More in the bottom right.
2. Tap the settings cog in the top right.
3. Tap Edit in the top right.
4. Update your Display Name and tap Save to confirm.
From a Leaderboard
1. Open any Leaderboard.
2. Tap the pencil icon next to your name.
3. Enter your new Display Name and tap Save.