Yes, if you have already filed and the return has been accepted, then it is too late to change the original return - you would have to amend the return.
Before you do that, though, look at the value of the car for donation purposes on the 1098-C that you received. Because the standard deduction has nearly doubled for most taxpayers, even if you add the 1098-C, would you still be under your standard deduction? If so, there is no point in amending your return, because it won't change your tax due or refund any.
If indeed you determine that adding the 1098-C would increase your itemized deductions, then please note that you will need to wait a bit to let the IRS finish the processing of your original return - we normally suggest waiting until you receive your refund or 3-4 weeks if you owe tax.
After that, please see the following link for information on how to amend your return with TurboTax: https://ttlc.intuit.com/replies/3288565
