Here is a step by step instructions on How to Change Your Youtube Username. Please Watch the Video Below for Clear Understanding.
Step 1: Go to YouTube's Website.
Step 2: Click on Your Username in the Top Right Corner of the Browser.
Step 3: Click on "YouTube Settings".
Step 4: Next to Your Username, Click on "Edit Profile".
Step 5: Click on Your Name on the Top Left Corner of Your Profile.
Step 6: In the New Boxes, Write Your Name as You Would Like It to Appear in Google+ and YouTube
Step 7: Click "Change Name".
Step 8: Go to YouTube and See Your New Name for Yourself.