On one of the first pages there is some sort of code to do this. But if you did not copy it you are out of luck. And if you try to open a new one you will get a message at some point that a duplicate passport number or flight number is already in the system................
I did my eticket too early and didn't copy the code. Called immigration at the airport and they said just do another one, no problem. I did just that and had no problem filling out a new form and traveling through the POP airport.