To edit a line, highlight it and select Edit from the left menu. This will bring up an edit screen where you can change details such as price or quantity. These changes are only applicable to the current transaction. You can also customer order the line, browse through supersessions or check stock in other stores.
The edit screen may com up automatically if set on an item to prompt for price or qty, or if set in settings to come up every time or if there is insufficient stock.
If you want to change the price but want it to show as a discount then use the Discount option on the left menu (If operator security allows).
If the line is a unit you can change the allocation, if it is a trade-in you can change the Trade-in details.