Click on the Item in the grid so the information for that item displays in the Item Inspector.
If you are having trouble finding the item you need to change, please see the section about Searching for Items. If you still cannot find the item make sure all filters are off - you should NOT see this on the bottom of your window. To clear this please see Filtering Items
Click the save button. This saves the new price to My Pricebook and marks this item to be uploaded to the register. At this point the register does not yet have the new price.