While your lump sum may include benefit payments for more than one year, you do not have to file an amended tax return for any prior year. 

You might need to pay taxes on a small portion of your lump-sum payment. However, you might be able to avoid these taxes by apportioning prior year benefits to those previous years’ income (still without filing any amended returns). 

After you enter your SSA-1099 information, we'll ask if you received a lump-sum Social Security payment, answer Yes and follow the onscreen instructions. Your SSA-1099 lump-sum section breaks down the payments into what amount was received for each preceding year. You can only enter one year at a time; when finished, click Add Another Lump Sum if you need to enter benefits received for a different tax year.