How can I change windows 7 from enterprise to ultimate edition after installation?

The only difference between the two is how they are licensed. Is there a feature you are trying to enable? I guess I am looking for a problem to solve... but don't see one. Maybe more detail about what you hope to achieve will help.

Because I got Ultimate edition serial number :)

According to MS, every edition is on every DVD. It is the Key. Some features may not be available if your hardware will not support them. If you have Win 7 Enterprise Full version up and running, I would advise you to save the key for a rainy day. If you are trying to convert the Evaluation Version MS does not support it.