Why did it reject? If it rejected it was never filed so you do not amend. You just fix it and print and mail it now.
If you need to fix something you will need to switch to the Desktop program. You need to use a full Windows or Mac computer. And mail the return. If you paid for the Online version ask support to give you the Desktop program download.
How to finish a prior year online return
https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-finish-a-prior-year-return-that-i-...
After you get the program installed the first thing to do before you open your tax return .tax file is to update the program and install any state programs you had. Then open your file. So you first might need to start a fake return to be able to download the state program (go to FILE - NEW).
