To over ride a unit price is simple. If you notice the unit price area has a red field background as shown below, it means that the user logged in does not have the authorization to override the price.  To do so they will need a manager or someone with the authority to override the price.
NOTE: If you are logged in with user authority to override a price the background color will be yellow

To override the price, double click on the Unit price.  If the background color was red, a window will pop up asking for an authorized person to enter their username and password to allow for a price change.

Once the override is successful it will open another pop up and give you the following message:

Click the done button to proceed to overriding your price.
You will now see that the background of the unit price is yellow, simply double click the price to edit the price.

Once the price has been changed, hit enter to continue. You will notice the price will now show a red asterisk to the right of the price indicating that this price has been adjusted.