I filed my taxes on my desktop computer at home, but I am out of town and need a copy. How do I get a copy of my already filed tax forms through the app

If you used desktop software the tax return is stored only on your own hard drive.  It is not stored in an online account.  

It's not clear whether you filed your tax return with the desktop TurboTax software installed on your desktop computer, or with TurboTax Online that you signed into using the web browser on your desktop computer.

If you used the desktop TurboTax software your tax return is stored only on the computer that you used. There is no way to access is remotely unless you have remote access to your desktop computer at home.

If you used TurboTax Online you can sign in to your TurboTax account from any computer that has internet access and get a copy of your tax return.

(You are using the term "app" very loosely. It usually refers to software that runs on a mobile device such as a phone or tablet. There is a TurboTax mobile app, but you could not have used the TurboTax mobile app on a desktop computer.)