You can’t even do a practice return without having the software installed. Each year‘s version of TurboTax is a separate stand-alone program. If you purchased them as downloads, you may be able to download them again from the vendor where you purchased them. If you purchased them on CD, and you uninstalled the program and threw away the CD, you will probably have to buy the program again if you want to amend.
As a rough guide, if you are married filing joint return with your spouse, and your taxable income after subtracting all of your other deductions was less than about $75,000, then adding additional charity donations you would return you about 12%-15% of the value of the donation. If your taxable income after subtracting all other deductions was more than $75,000, adding additional donations would return about 22% to 25% of the amount of the donation. In other words, if you forgot to list a $100 donation, your additional refund if you filed an amended return would probably be between $12 and $25, depending on your income and the year.
