How to change the Display Name at Top Right Corner

Summary

Requirement is that the Display Name should be ( 'Known As' + Last Name). If employee has no Known As name it should display First Name + Last Name

Content

The client wants the Display Name which is shown on top right corner to show (Known As + Last Name) and if the user does not have a Known As name it should display First Name + Last Name.

For Example :

1) First Name: Michael

Last Name : Walter

Known As : Mike

Home page should Show : Mike Walter

 2) First Name: Thomas 

Last Name : Cook

Known As : 

Home page should Show : Thomas Cook

We looked as the customization(source) code on this field as currently it is #{GlobalAreaBackingBean.userDisplayName}