How do I change from the itemized to the standard deduction on my state return after it has already been filled out but not submitted?

The following TurboTax support FAQ is for changing to Itemized from Standard, but it can also be used for changing from Itemized to Standard - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-credits-deductions/change-standard-deduction-itemized-vice-versa/L7evWtHhv_US_en_US

Note - If the total of your itemized deductions are less than the Standard Deduction for your filing status the Standard Deduction will be used.

Standard deductions for 2024

Single - $14,600 add $1,850 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

Some states will allow you to use a different deduction from your Federal return, but others require you to use the same deduction as you did on your Federal return.  You need to be sure that your state allows you to not match the Federal deduction.  

Then, you will most likely need to change the deduction used on your Federal return by using the steps supplied by the article shared by DoninGA.  Once the Federal return deduction is changed, you will need to start your state return again to transfer the new deduction choice to the state return.  Then, if you are planning to file the returns at the same time, you will need to go back and change the Federal return after you have completed your state.

If you are not sure whether your state will allow you to use a different deduction than your Federal return, you can share what state you are filing and someone can verify whether a difference from Federal to state is allowed.

@hirst-le