Do NOT start messing around with your already filed tax return that was prepared using online software or you will mess it up permanently.
If you want to "play" with what-if scenarios, start a NEW return and just do not go all the way through the steps to actually pay or file it.
AND...for the Online software....starting a NEW tax return means you create a new account, from scratch, with a new UserID and Password.
BUT, keep those two accounts straight as to which contains your real tax file.
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If you want to do what-if's regularly, you should buy and use the Desktop software to be used on a full Windows or MAC computer. The desktop software allows you to create as many separate test files as you want, and also allows you to see ALL the forms at any time without paying extra.