If you want to correct your application and your deadline hasn't passed yet, it is possible to withdraw your current one and resubmit a new one before your deadline. This is what the NSF told me:
"Once an application has been submitted, it is not possible to edit any of the content, other than your contact information.
If your field of study deadline has not passed, one possibility is to withdraw your submitted application and submit a new one by the appropriate deadline. You can withdraw your application by logging into your GRFP account on FastLane and selecting the "withdraw application" option. You will then need to create a new application account and re-enter all your information. All reference letters and transcripts must be resubmitted by the field of study deadline: application components cannot be moved from one application to another.
Applicants are limited to one submitted application in the 2019 GRFP. If you plan to start a second application, YOU MUST WITHDRAW THE OLD APPLICATION by your field of study deadline. Applicants who have more than one submitted 2019 GRFP application by the deadline will have all submitted applications for this year's competition returned without review.
Please be aware that starting a new application in order to make corrections to a previously submitted application does not afford an applicant any special extenuating circumstances. Applicants who are considering withdrawing a submitted application and submitting a new one are advised to carefully consider factors such as the amount of time before the deadline, whether they can successfully re-enter all necessary information before the deadline, whether any references with submitted letters will be able to resubmit their reference letters for a new application prior to the November 2rd deadline, etc."