How do I file Form 1040X for 2018 due to VA retroactive disability payments?

[DoninGA]

Why do you think you need to amend a tax return for the Veterans Administration disability payments you received?  VA disability payments are not reported on a tax return.

Or are you referring to this - 

Disabled veterans may be eligible to claim a federal tax refund based on:

• an increase in the veteran's percentage of disability from the Department of Veterans Affairs (which may include a retroactive determination) or
• the combat-disabled veteran applying for, and being granted, Combat-Related Special Compensation, after an award for Concurrent Retirement and Disability.

Go to this IRS website for information - https://www.irs.gov/individuals/military/special-tax-considerations-for-veterans

And this IRS FAQ - https://www.irs.gov/individuals/military/frequently-asked-questions-regarding-disabled-veterans-pens...