When you've earned titles in QuizUp you can choose which one you want to display at any time. You start earning titles when you surpass level 5 in a topic.
When you've opened the QuizUp page (QuizUp tab), located in the middle of the tab bar you should see the Settings option located in the left corner on the top bar.
To change your title:
Tap the settings option in the top left corner
Under settings you'll find the Edit profile option
There you should be able to choose between all your earned titles
Tap save in the top right corner
From QuizUp.com simply click the triangle besides your display name and select settings. From there you can edit your profile and select a new title.