how can i change the federal tax pin on my form in turbotax

Which PIN are you referring to?   

• The IRS issued 6-digit Identity Protection (IP) PIN - How do I get my 6-digit IP PIN and enter it in TurboTax?  You can't reuse it; the IRS gives you a new each year.
• The self-issued 5-digit Self-Select (SS) PIN that you create every year when you e-file.  Each year you create a new SS pin that you can use as an e-file verification method next year.  You can use the same SS pin each year or make up a new one, but only you know what numbers you entered.   It is not saved by TurboTax.

To e-file this year, you need last year's:  Either Self-Select pin or AGI.  Then when you e-file next year, you will use this year's AGI or the SS pin you enter during filing this year

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Everyone that e-files has to use the 5-digit SS pin, but only certain individual are issued an IP PIN.  

Is my Self-Select PIN the same as my IP PIN? NO

Can I reuse last year's IP PIN?  NO

You can reuse your Self-Select PIN each year