Yes, you will have to file an amended return. Since the IRS also gets a copy of 1099-DIV which you received as well, they will know that you did not enter the Dividend Income. They will assess interest and penalties if you fail to report all the income.
Amended returns can be e-filed or it can 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.
The option to amend a 2021 tax return will become available later in the tax season.
