I filed my taxes and it showed me getting 679 back and when I was done filing I only got back 431. Why? And can I cancel my filing and start over?

It is because as more information you enter in the program, the numbers change.  At the end of filing, it shows you have lesser total withholdings from your wages along with other deductions and credits that can cover your total taxes due.  That's why you are receiving a lower refund.  As long as you have not submitted your return by hitting that "Transmit" button at the end of the process, you are not considered as filing your taxes yet.  

If you want to cancel out and start over to just double-check, here are the steps:

• Sign in to your account, select Pick up where you left off
• To your left, select Tools, click on the drop-down menu
• Select Clear and Start Over
• Then re-enter the information