Generally speaking, you opt out by not having on Schedule 3 line 12e an amount that is greater than $0.
Note that if you pay enough in tax withholding and estimated taxes to cover your tax liability including the optionally deferrable SE tax, you cannot get it refunded so as to be able to defer it.
TurboTax calculates the amount that you are eligible to defer and then asks how much of that to defer. TurboTax places on Schedule 3 line 12e the amount that you specify.
