How do I switch to the standard deduction after I have already started with itemizing?

Your return will use the standard deduction until you have entered enough information about itemized deductions to exceed the standard deduction.  When you leave the Deductions and Credits section of your return by clicking Done with Deductions at the bottom of the screen, you should see a screen that will tell you whether the standard deduction or itemized deductions is better for your situation.  On that screen, you can keep the TurboTax recommendation or change it.  

So, if you stop entering itemized deductions, or do not have enough itemized deductions to be a benefit, then the standard deduction is automatic and you do not need to change to it.

(The only exception to this is a case where you are using the Married Filing Separate filing status and you are required to itemize deductions because your spouse itemized.)