Once a title is unlocked, it can be selected within the Player Status menu. Your selected title is displayed next to your name in menus and during duels.

Unlockable	How to unlock
Bestower of Boons	Give a creature you control +10/+10 or more.
Disciple of Marit Lage	Control a creature that's 20/20 or bigger.
Lord of Legends	Control five legendary creatures.
The Impaler	Sacrifice a creature.
The Traveler	Visit every plane.