Since the 2020 tax return was rejected by the IRS nothing for 2020 was received by the IRS, it can now be changed, completed and filed again.
To complete and file a 2020 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2020 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2020 tax return can only be printed and mailed, it cannot be e-filed.
If you're referring to your 2020 tax return, there's nothing to cancel. If it was rejected, then it was never e-filed with the IRS. Maybe you printed and mailed your 2020 tax return to the IRS? Can you clarify please, just exactly what the issue is with needing to do anything with your 2020 tax return?
Or do you mean your 2021 return for last year?
For 2021 you can try to clear and start over. But if you filed and it rejected it probably won't let you clear it.
Or if it doesn't let you clear you could just start a new account. Online is only good for 1 return per account. But you can use the same email address for 5 accounts. How to start another return in the Online version
Did it reject because you were claimed on someone else's return? Is that the only thing you need to change? Or what else is wrong? If you change that you are a dependent that might be enough. You still have to report all your income.
For this years 2021 return go to My Info and look for the box that asks if you can be claimed on someone else's return.