Go to the transaction we19.
Give the existing idoc number.
Click on execute.
go to the particular segment in whcih you want to chnage the values.
Double click on that particular segment,Here you can keep the values thta you want or chnage the existing values.
Click on ok.
click on tab standard inbound or standard outbound depending on the requirement.
The new idoc will be created with the updated information.
Please let me know if you need further info on this.