After you file

"How do I file older returns and amend them?"   

You do not do both.   If there is a past tax year for which you have not filed a return, then you prepare the return and mail it in.  You do not "amend" it if it is the first time you have sent in a return for that year.  Amending is a special process you use if you already filed a return that has been fully processed by the IRS but that has incorrect or missing information that needs to be corrected.

Please explain what tax years you are asking about.   Did you file some returns for past years that need to be corrected because you left out some information or made mistakes?  

Or do you just have some past years in which you did not file tax returns and need to get caught up?

It might be better for you to seek local paid tax help in a situation when you are several years behind in filing your tax returns.  Do you even have all of the documents you will need to enter and show your income—like your W-2’s and any 1099’s?

Each tax year has to be filed separately using the forms  and software for the specific tax year.  They cannot be combined in any way--do not even put them in the same envelope when you mail them.

TurboTax has software for 2020 onward.   Software is no longer available from TurboTax for earlier years, nor would it be supported even if you find it on eBay, etc.  Updates are not available.   It is also too late to receive a tax refund for a tax year earlier than 2020---those refunds have been forfeited.   BUT if you owe tax due you still have to pay with penalties and interest.   There might be other software providers that have software for those years, but you will not receive refunds for the returns even if the forms show a refund.

If you have not filed a tax return for 2020, it has to be filed by May 17, 2024 or you will forfeit that refund.

You cannot change the tax year.   The current online program is for 2023 only.   Only a 2023 return can be prepared online and only a 2023 return can be e-filed.

Online preparation and e-filing for 2020, 2021, and  2022 is permanently closed.

To file a return for a prior tax year

 If you need to prepare a return for 2020, 2021, or 2022  you can purchase and download desktop software to do it, then print, sign,  and mail the return(s)

https://turbotax.intuit.com/personal-taxes/past-years-products/

You may also want to explore purchasing the software from various retailers such as Amazon, Costco, Best Buy, Walmart, Sam’s, etc.

Remember to prepare your state return as well—if you live in a state that has a state income tax.

https://ttlc.intuit.com/community/state-taxes/help/how-do-i-contact-my-state-department-of-revenue/0... 

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

Federal and state returns must be in separate envelopes and they are mailed to different addresses.  Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.

Note:  The desktop software you need to prepare the prior year return must be installed/downloaded to a full PC or Mac.  It cannot be used on a mobile device.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

Federal and state returns must be in separate envelopes and they are mailed to different addresses.  Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.