Insert server 2008 CD into CD/DVD drive. Then right click CD/DVD -.> explore. Open the source ->adprep copy folder , And paste the c: drive, Then go to run->cmd ,Type in cd\ and hit Enter .To verify that the adprep folder is on your hard drive, type in dir and hit Enter.

And check adprep folder

Next, type in cd adprep and hit Enter

Next type is adprep /forestprep then hit Enter.

Next adprep /domainprep and hit Enter.

Domainprep will now run and once it is done you will receive a confirmation.

Close Run .

Then go to setup file and double click.

Then select the upgrade option and then select Next.

Your Server is now being upgraded.