Cost and charge rates are assigned to resources and used to calculate the cost and chargeable value of timesheet entries. If you subsequently change a rate, it will only be applied to future timesheets and any with the old rate will be unaltered.
Select Adjust Charge Rates or Adjust Cost Rates from the Financial section of the Administration menu if you want to apply rate changes retrospectively to timesheets or if you want to use a totally different code and apply its rate to previous timesheets.
Input is over two tabbed forms and is the same for both costs and charges. Click 
on either one after making your settings.