This page has a guide on how to change your in-game name in DayZ Standalone (DZSA).

Step 1: Go to the main menu and click on your name in the bottom right corner. (where red arrow is pointing to below)

Dayzsachangecharname1.jpg


Step 2: Enter the new name using your keyboard:

Dayzsachangename2.jpg


Step 3: Click anywhere outside the name area, until the cursor disappears

Dayzsachangename3.jpg