want to make traditional IRA contribution for 2015; already filed tax return

[Cheddar Bob]

I want to open up a traditional IRA and contribute the max for 2015.  I believe I have until April 18 to do so.  However, I have already filed my 2015 tax return and received a small refund.  I know one of the benefits of contributing is that it counts as a deduction for AGI.  Is it possible for me to file an amended 2015 return?  I filed online using H&R Block, if that matters.

I already contributed the max to my 401k in 2015.  I'm aware that there are potential deduction limits for IRA contributions depending on my AGI, but my AGI is low enough such that I should get the full deduction.  IRS table is here: https://www.irs.gov/Retirement-Plans/2015-IRA-Deduction-Limits-Effect-of-Modified-AGI-on-Deduction-if-You-Are-Covered-by-a-Retirement-Plan-at-Work

[seattlecyclone]

Yes, you would need to file an amended return.