After filing my 2018 taxes I received a letter from the IRS saying I owe them more money.

[lynn68]

How do I amend my tax return and contribute to my SEP to lessen that amount due?

[KittyM]

Be certain that you reply to your letter from the IRS, especially if they have suggested adjustments to your return.  Either agree with their changes or not.  Sometimes, you can make your proposed changes via reply to this letter, using your amended return as a support document, rather than filing the amended return.  It's much faster if you can do it that way, and less confusing to the IRS.  

Here is a link to the TurboTax Audit Support Center if you need further help in dealing with your IRS notice.  And here's how to Amend My Return in TurboTax.