It depends, if you received a health insurance form 1095-B or 1095-C and answered in TurboTax that you had health insurance all year then there is nothing that you need to do since the IRS does not need any details from these forms.
If you forgot to answer the health insurance question or received a different form mentioned above then it depends if your return has been rejected or accepted.
If your return has been rejected by the
IRS, all you will have to do is make the changes on your return and refile.
If your return has been already accepted
by the IRS, you will have to file an amended
return (1040X) to include the form 1098-T. But you might want to wait until
your refund or payment is processed. The IRS will only accept amended
returns via mail.
The 1040X form for 2018 hasn't been approved yet
by IRS. Please, keep checking the following information on how to amend your
return; it will be updated once the 2018 form is released:
https://ttlc.intuit.com/replies/3288565
