I entered my savings account routing number and account number on my tax return, but accidentally checked the box saying it was the checking account. What do I do?

Assuming that you have already filed on you can't go back to the File tab in TurboTax and make the adjustment, you won't be able to change that selection after the return is sent.

In all likelihood, your bank will still accept the deposit as long as the account and routing numbers are correct and this shouldn't be an issue.  That is what really matters for the transaction.  However, it's possible your bank may decline it and if so, the IRS would then mail a paper check to the address on your return instead.