If you have not yet submitted your return, you can update/edit your Social Security Number (SSN) by going to the Personal Info section (click on the personal info tab on the top of the screen) and click on edit next to your information.
If you have already transmitted/efiled your return, it is very likely that the return will get rejected, at which point you will receive an email informing of the rejection and providing you with instructions on how to fix the return.
If for some unlikely reason the return does not get rejected, then you will have to amend it. If you must amend the return.
As a courtesy, I have included a link for information on how to amend a 2017 tax return.
To amend a 2017 return, follow the steps in the article below (The 2017 amended tax return instructions will be available sometime in late January/Early February):
I put wrong social security number in for 2018 how do I correct
So have you already filed your 2018 return?
If so, you will need to amend your return.
To learn more information on how to amend your return, please see this TurboTax FAQ.
Have you already filed and completed your return? @Kalv