How do I enter student loan interest? What is AGI phase out for deductibility?

To enter student loan interest paid -

• Click on Federal Taxes (Personal using Home and Business)
• Click on Deductions and Credits
• Click on I'll choose what I work on (if shown)
• Scroll down to Education
• On Student Loan Interest Paid (Form 1098-E), click the start or update button

Or enter student loan interest in the Search box located in the upper right of the program screen. Click on Jump to student loan interest

Student Loan Interest Deduction
You can take this deduction only if all of the following apply.
• You paid interest in 2019 on a qualified student loan (defined later).
• Your filing status is any status except married filing separately.
• Your modified adjusted gross income (AGI) is less than: $85,000 if single, head of household, or qualifying widow(er); $170,000 if married filing jointly. Use lines 2 through 4 of the worksheet in these instructions to figure your modified AGI. 

• You, or your spouse if filing jointly, aren't claimed as a dependent on someone else's (such as your parent's) 2019 tax return.
Use the worksheet in these instructions to figure your student loan interest deduction

IRS Form 1040 Instruction page 90 for the worksheet - https://www.irs.gov/pub/irs-pdf/i1040gi.pdf#page=90