Amending returns that were filed by my EX-Accountant, multiple years... ISO HELP

Hello

Thanks for any advice on this situation!

 

I'd like to amend my taxes as far back as I possibly can...

My accountant was doing them incorrectly and not include some crucial information. 

 

1- How many years back can I amend?

2- Should I use turbo tax and start the tax process for those years over from the beginning? 

 

Thx again