What did you get it for? Unemployment or a state tax refund? If it is a state refund it might not be taxable and you might not need to report it.
It is taxable if you itemized deductions on that year's federal return and took a deduction for state income taxes instead of the sale tax. You got a deduction benefit for it so now you have to include it as income. If you took the standard deduction it is not taxable.
You first need to wait until the IRS Accepts or Rejects your return. If they reject it for some reason you can just fix it and resend it.
If it was Accepted, you need to wait for your return to be fully processed and you get the first refund or your tax due payment has cleared. Then you can mail in an amended return. Do NOT change anything yet on your return until you start the amended return. If you have changed it then take out the changes and put it back to the way it was when you originally filed. You can fill out the amended return now but it won't be ready to print it out until mid-Feb
How to Amend
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed You will get your original refund and get to keep it. When you go through the amendment section it asks you how much you paid or got back on the original return so be sure to enter it. Then the amended return will only show the difference you get back or need to pay. You can confirm this by printing your amendment and looking at the 1040X. Line 17 should be your original refund amount and line 20 should be your additional refund. If you paid on your original return it will be on line 15. If there is an amount owed with the amendment, it will be on line 19.
