She can still be your dependent if she is under 24 and a full time student. In fact on her return she has to say she CAN be claimed on someone else's return even if you do not claim her. So I don't think you need to amend.
Are you using the Desktop installed program or the online version? The return should be saved. How did you try to save it? Online automatically saves returns unless you do another return in the same account then it only saves the last return. Online is only good for 1 return.
If you used the Desktop CD/Download program you should have a file ending in .tax2020 on your computer. We can help you find it.
