I need to change my download type from Windows to Mac.

No problem. Please contact us using the instructions below. Yes, it is a process; however, it will get you there! An agent can verify your Windows purchase and push you a Mac download.

https://support.turbotax.intuit.com/contact/

I need to change my download type from Mac to Windows.

This FAQ has directions to contact a specialist that can push a download to you.

You will need to fill out some information first. 

https://support.turbotax.intuit.com/contact/ 

You can also call TurboTax support through the article below.

What is the TurboTax phone number?

I called the help line and they said that they could not help because I bought my 2020 from a third party, Amazon.

Is this correct that third part purchases are excluded from support?

Since you bought from a third party you need to go back to the third party and get a refund of the product you cannot use and purchase the product you can.