The Time Periods are locked at the completion of the Forecast Configuration wizard, and so are the Ranges once the first Save Draft or Commit action is performed within the Forecasts module.
If the selections you made for Time Periods and Ranges need to be changed, the only way this can be accomplished is at the expense of losing any existing Forecasting data.
If you want to reset the forecast settings to change the configuration then please see the following Get support in Access CRM and reference the title of this article.
