I received 1099S for the sale of my house after I filed. I used the money to pay off the mortgage & put a down payment on my new house. Do I amend now? or after refund?

You can not add the form/information to your original tax return once it has been filed AND accepted by the IRS.  If it gets rejected, however, just log back in to your tax return add the form and make any other necessary corrections. 

If accepted, you will need to file an amended return (1040X) to show the additional form/information. You should NOT try to amend your return until the original return has been fully processed and you have received your refund or your payment has cleared.

Amended returns do not “catch up” to your original return and replace them; they are processed as two separate returns.

Amended returns have to be printed and filed by mail.  It can take the IRS up to 12 weeks or longer to process them.   If you are due a larger refund than on your original return, your amended return should only show the difference and you will receive a separate check for it.  If you owe money on your amended return, it will show only the new amount owed, you will have to mail a check with the return.  Three weeks after you mailed your amendment, you can start tracking its progress at the IRS Where's My Amended Return? site.

Please read this TurboTax FAQ for instructions on How do I amend my 2017 TurboTax Online return?

Please see this TurboTax FAQ for more information about entering a 1099-S (Proceeds From Real Estate Transactions): https://ttlc.intuit.com/replies/3302480

[Edited 2/26/18 | 10:09am]