If you are needing to make a correction or changes to an already filed and "Accepted" by the IRS return you can do that. If you are wanting to cancel an already filed return, once it is e-filed you cannot cancel it unless the return is rejected.
If your return was rejected you have two options: Correct the return and re-file or do nothing and it will not be filed. If the return was rejected, you will receive an email with the rejection code, an explanation, and the instructions to correct the issue.
What does it mean to "amend" a return?