From this option you can easily change the rates of sales invoice while making the bill or after you have saved the bill.
To execute the following process you need to make bill in sales invoice as shown in the image.
Now as shown in the image above, if you want to change the rates in this bill you have to click Mouse right button and select Change Rate option shown in the image below.
Now as you can see in the image above you can apply the following rates change in the bill.
You can change these rate as per (MRP/ Sales rate / Trade rate/ Distribution rate/ Purchase rate/ Cost Rate) and click on the SAVE button
So now you can either select a single product or tick-mark Apply for all items. So whatever option you will select rate will change according to that selected option and the rate applied on lots of that item.