1. Login WHM

2. Go to  WHM  » IP Functions » Assign IPv6 Address

3. Click on Account that need to assign IPv6

4. Click drop down in right box to select range of IPv6. Example to choose "The System's shared IPv6 address."

5. Click on Enable Account for assign IPv6 to Account.

6. When go to WHM  » IP Functions » Assign IPv6 Address  it will show as below.