How to amend my return if I forgot to select the standard deduction?

Why do you think you had to select standard deduction?   Unless you entered enough itemized deductions to exceed your standard deduction, the software defaults to standard deduction.  Further, your question is posting from the Free Edition, which does not support entering itemized deductions.    

Look at your Form 1040.  You will see your standard deduction on line 12.

The Standard Deduction is automatic unless you have entered itemized deductions which is greater than the Standard Deduction for your filing status.

If you are a dependent and your taxable income is less than the Standard Deduction for your filing status then that income is shown as your Standard Deduction.

The Standard or Itemized deduction is shown on your federal tax return, Form 1040 Line 12

Standard deductions for 2024

Single - $14,600 add $1,950 if age 65 or older
Married Filing Separately - $14,600 add $1,550 if age 65 or older
Married Filing Jointly - $29,200 add $1,550 for each spouse age 65 or older
Head of Household - $21,900 add $1,950 if age 65 or older

Are you a dependent?  Then your Standard Deduction on line 12 might be less than the table.