How can I download my 2018 tax return? I need this document on file in order to refinance my home.

If you're in the same TurboTax account that you used last year, you can find your 2018 return in the Documents tab: 

1. Select Documents from the left menu.
2. Use the drop-down at the top of the screen to choose the tax year you want and then select Download PDF.

More detailed info here: https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/00/27010