Should I wait until I receive my paper check to amend? How am I supposed to do after I receive the paper check?

I am a J1 visa holder, and I've filed my tax return for 2018 with Turbotax and got accepted (my paper check is on its way). 

However, I realized that J1 visa holders could not use Turbotax to file, and I would like to amend. I think I need to wait until I receive the paper check and mail a new form to IRS, but should I put the paper check that I receive from IRS in the envelope to send it back with the new form? 

Also, do I have to amend by April 15, 2019 ? 

Thank you