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. 

iOS/Android/Windows Phone:

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:
  1. Tap the settings option in the top left corner
  2. Under settings you'll find the Edit profile option
  3. There you should be able to choose between all your earned titles
  4. Tap save in the top right corner

Web:

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.