If I made an IRA deposit after I filed my tax return and before July 15, 2020, will it reduce my taxes owed?

[macuser_22]

Only if it was a Traditional IRA contribution made for 2019 and you were eligible for a deduction.    You must amend if already filed to report it.

The maximum IRA contributions for 2019 is $6,000, or $7,000 if you’re age 50 or older by the end of the year; or your taxable compensation for the year which ever is less.

(Taxable compensation is generally wages that you worked for - W-2 or net self-employed income minus the deducible part of the SE tax, but can include commissions, certain alimony and separate maintenance, and nontaxable combat pay ).

See IRS Pub 590A "What is compensation" for details:
https://www.irs.gov/publications/p590a#en_US_2018_publink1000230355

See this IRS link for Traditional IRA deduction limits when covered by a retirement plan at work.

https://www.irs.gov/Retirement-Plans/IRA-Deduction-Limits

[fanfare]

did you take a deduction for that on your tax return?