Yes, you would need to file an amendment if the new information creates a change in your refund or amount owed. You cannot e-file an amendment, therefore you would need to submit a Form 1040X and include a copy of your Form 1042-S to IRS. Please continue to read:
You have two options:
If your return got rejected for any reasons, you can return to the program/account to add the info along with other changes. You do not need an amendment 1040-X.
If your return has been accepted, please see the following :
You will have to amend your return by filing a Form 1040X. You should not try to amend your return until it has been fully processed and you have received your refund or your payment has cleared. If by editing the information does not change the refund or amount you owe, you do not need to submit the amendment.
Amended returns do not “catch up” to your original return and replace them; they are processed as two separate returns.
Amended returns have to be printed and filed by mail. It can take the IRS up to 12 weeks or longer to process them. If you are due a larger refund than on your original return, your amended return should only show the difference and you will receive a separate check for it. If you owe money on your amended return, it will show only the new amount owed, you will have to mail a check with the return.
https://ttlc.intuit.com/replies/6603004
Per IRS, code 29 on a Form 1042-S is for reporting interest from your bank accounts. You would treat it as if you receive a Form 1099-INT
In the amendment, to add your interest amount, simply follow these steps:
1. Sign into your account, select Take me to my return
2. In the upper right corner, in the search box, type in 1099int
3. Select Jump to 1099int
4. On screen, Did you receive any interest income? answer Yes to continue to follow prompts
5. On the screen, Let's get 1099-INT or brokerage statement details, enter the amount in box 1
( check box for My form has info in more than just box 1 if it applies to you)
