If you want to change your method of accounting for inventory, you must file Form 3115. Unfortunately this form 3115 is not supported by TurboTax for efiling. You can print it through TurboTax. If you think this might not be correct go back and edit your inventory questions in your business. Carefully read the questions about inventory.
Good information about inventory:
If you have more questions about this, ask in the comment box below.
Here is the form from the IRS:
Here are the instructions: