To Modify an Existing Entry - Click On the Item you want to Modify - This entire line will highlight (see image below)

Click the button MODIFY - This will open the Modification Dialog (see image below)

IMPORTANT this is for single modifications - If you choose more than one entry at a time, use the Global Modification function.

ONCE DIALOG IS OPEN - Make your changes to this input Item and then click ENTER to apply the changes 

REFRESH - You MUST refresh the bill after a modification - Click on the Button below the Global Modification Button to Recalculate Bill