Unfortunately, Form 3115 is out of scope for TurboTax. My best advice is to see a professional preparer if you don't feel you can fill out the form without help.
Below are the IRS links related to the change in accounting method. TurboTax does not have that form.
Maybe these will help
Form 3115, Application for Change in Accounting Method
Change-in-Accounting-Method
Instructions for Form 3115 (03/2012)
Instructions
Form 3115,
Form 3115
