I highly recommend you use the superior  downloaded version going forward so you ALWAYS have access to a working program to what if to your heart's delight.   

SAVE this info for later ....

If you prepared your 2019 return online, in order for your previous year tax information to transfer into your Desktop software you need to download your 2019 Online return as a .tax2019 file. 

https://ttlc.intuit.com/questions/1899645-how-do-i-save-my-2015-turbotax-online-return-as-a-tax-data...

If you can't get or didn't save the .tax file to your computer before online was shut down Oct 15 or lost it,  there is a way to do it by starting your return in the 2020 Online version, transfer into that and then you can switch and continue in the desktop version and you won't have to pay for online.  Expand the 3rd section for transferring an Online return in this FAQ
https://ttlc.intuit.com/questions/1901506-transfer-last-year-s-return-into-turbotax-for-windows

This is my mini version of  a tutorial  that should be in the downloaded program: 

Always remember to shut off the auto update function once you have filed your returns and do not do any updates unless you have safeguarded the PDF & .taxfiles to a removable storage unit for safekeeping.

What is Forms Mode?

Forms Mode lets you view and make changes to your tax forms "behind the scenes."

If you're adventurous, you can even prepare your return in Forms Mode, but we don't recommend it. You may miss obscure credits and deductions you qualify for, and you may forget to report things that will come back and haunt you later.

Forms Mode is exclusively available in the TurboTax CD/Download software. It is not available in TurboTax Online.

Related Information:

• Why would I use Forms Mode?
• How do I switch to Forms Mode in the TurboTax for Windows software?
• How do I switch to Forms Mode in the TurboTax for Mac software?

If you want to play around with different figures and tax scenarios without affecting your original return you can ….

• >>>In the TurboTax CD/Download software by creating a test copy: 
• 1.  Open your return in TurboTax. 
• 2.  From the File menu, choose Save As. 
• 3.  Give the copy a new name to distinguish it from the original (for example, by adding "Test" or "Example" to the file name). 
• 4. Click  Save. You are now safely working in the test copy and anything you do here will not affect the original. 
•  https://ttlc.intuit.com/questions/1900642-how-to-make-a-test-copy-of-your-return

• >>  use the WHAT IF tool: 
• - Click Forms Icon (upper right of screen) or Ctrl 2 (forms view) 
• - Click on the Open Form Icon 
• - In the “Type a form name.” area type What-If (with the dash), click on the name of the worksheet - click on Open Form 
• - You will see the worksheet on the right side of the screen; enter the information right into the form 
• - To get back to interview mode - click on the Step-by-Step Icon (upper right of screen) or Ctrl 1

It's always a good idea to make a backup copy of your tax data file, in case your original gets lost or corrupted. Here's how:

1. From the File menu in the upper-left corner of TurboTax, choose Save As (Windows) or Save (Mac).
2. Browse to where you want to save your backup.
   • Tip: If you're saving to a portable device, save it to your computer first to prevent data corruption. Then, after completing Step 4, copy or move the backup file to your device.
3. In the File name field, enter a name that will distinguish it from the original tax file (for example, add "Backup" or "Copy" to the file name)
4. Click Save and then close TurboTax.
5. Restart TurboTax and open the backup copy to make sure it's not corrupted. If you get an error, delete the backup and repeat these steps.

If you make changes to your original tax return file, repeat these steps to ensure your original and backup copies are in-synch.

Related Information:                             

• Retrieve a Tax File from a Portable Device
• What's the difference between the tax data file and the PDF file?

AND save it as a PDF so you have access to a copy even if you don’t have the program still installed and operational :

• How do I save my return as a PDF in the TurboTax software for Windows?
• How do I save my return as a PDF in the TurboTax software for Mac?

AND protect the files :

• How can I protect the tax files on my computer?

Here's how you can see a list of all your overrides (if any). On the Tools menu select My Tax Data. A window will open that lists all the entries in your return. Any entry that was overridden will say Overridden in the Source column. You can click on the heading of the Source column to sort on that column, so that any Overridden entries will be together. If there are Details or Import entries at the top of the list after you sort it, scroll down to see if there are any Overridden entries after the Details and Import entries. The item in the My Tax Data list tells you the form and line number that has the override. You can click on an overridden item in the list, then click the Go To Form button at the bottom of the list to go to the form that has the override.