Capital losses (short-term or long-term) cannot be carried back to an earlier year. You report the loss in the year that you sold the stock. If the loss cannot all be used that year, it will be carried forward to subsequent years. Up to $3,000 of the loss will be used each year to offset other income (besides capital gains).
This has nothing to do with whether you use TurboTax Online or the CD/Download ("desktop") TurboTax software. How capital losses are applied is determined by the tax law.
