To edit the rate margins, you will need to have admin privileges. Go to Preferences at the top right of the main screen and select the Rates tab. 

Under this tab you will see all the prices, deposits, surcharges and costs used within your calculations. To edit, click on the blue Edit text to the right of each value. 

Choose a new amount and then select either Fixed, Miles or Percent from the drop down. Then hit Update to confirm. 

You can also view a history of previous rate changes to each value by clicking on the green clock icon to the right of the Edit button. This will tell you exactly when it was changed, the new and old values and which user made the change.