You will need to amend your 2015 tax return by April 15, 2019.
You have 3 years from the date you filed your return (or 2 years after you paid the tax due, whichever is later) to file an amended return.
I would print out a Form 1040X and complete it manually as the only thing you are changing is the statement. I would include the 1040X and the 2 page 1040 and the statement.
According to Publication 519, page 9:
Amended return. You generally make this
choice when you file your joint return. However,
you also can make the choice by filing a joint
amended return on Form 1040X. Attach Form
1040 and print “Amended” across the top of the
corrected return. If you make the choice with an
amended return, you and your spouse also
must amend any returns that you may have filed
after the year for which you made the choice.
You generally must file the amended joint
return within 3 years from the date you filed
your original U.S. income tax return or 2 years
from the date you paid your income tax for that year, whichever is later.
You should file the amended returns for 2016 and 2017 as well to include the statement.
Don't forget to sign the amended return before mailing and include on page one of the 1040X the year you are amending (at the top of the page).
For 2018, you need to include the statement with your 2018 return that you mail.
TurboTax does not calculate FICA taxes.
Publication 519
https://www.irs.gov/pub/irs-pdf/f1040x.pdf
