As the PP said, you can't do this on line "immediately" however, if you don't want to wait to make the name change on your reservation, you can send an email request and they will make the change and email you back when it has been completed.
The email process is slow. I recently did this and it took about 2 weeks for this request to be completed.
If you have any type of urgency to the change
DVC MS is the way to go.