Your filing status is entered in the My Info section of the program when using the online editions.
Click on My Info on the left side of the online program screen. Edit your Filing Status.
@joker2u64-gmail- just be aware, there is really no financial benefit and in fact it's normally a financial cost to file MFS (separate) instead of MFJ (joint). 96% of married couples file joint and those last 4% either have marital difficulties or just don't understand. You lose a lot of tax benefits by filing Separate and the tax when adding the liabilities of the two spouses together is higher than if they just filed joint.
I have been unable to find an example where there is a financial benefit of filing MFS (Separate).
Just be aware...
Only benefit might be an income-based student loan repayment program.
You asked how to change from Single. Be aware, that in some limited circumstances a Legal Separation might qualify you to file as Single.
@SweetieJean - the benefits of filing separate to 'save' on student loan payments is dubious.
While IBR payments may be lower when calculated solely on the one income, the other spouse is going to pay higher tax on the other income when using the MFS tax tables. The question is whether the higher tax bill for the family (since both are filing MFS) is offset by lower student loan payments.
And those lower student loan payments will result in a higher loan foregiveness after 20 years of payments. Post 2025, the loan foregiveness is reportable federal taxable income, with the exception of the PSLF repayment program which is covered by a separate federal law and the loan foregiveness is not taxable by the federal government, no matter when the foregiveness occurs.
So the savings of lower student loan payments plus the additional future taxes due to higher loan foregiveness has to be less than the income tax due based on both filing MFS.... that is so uncertain it is dubious that filing MFS will lead to a better outcome in the long run.