Hello,
If you are attempt to install DNS role to setup a new AD-Integrated Zone on this DC, then please follow the instruction provided by Sachin. If anything unclear, feel free to follow up with us.
Otherwise, do you mean that you have a DC which is also a DNS server, you would like to convert the existing Primary DNS Zone to a AD-Integrated Zone?
For Windows Server 2003 or Windows Server 2000, refer to:
How To Convert DNS Primary Server to Active Directory Integrated
http://support.microsoft.com/kb/816101
For Windows Server 2008 or Windows Server 2008 R2, you can configure the Primary DNS zone for use with the AD DS.
Refer to: Understanding Active Directory Domain Services Integration
http://technet.microsoft.com/en-us/library/cc726034.aspx
Thanks
Zhang