Each watering can upgrade allows you to water more tiles at once, and also increases the number of tiles that can be watered between refills. The copper upgrade lets you water 3 in a line. Just touch and hold the first tile you want to water, green squares will show you which tiles will be watered, and let go to water the plots. The steel upgrade waters 5 in a line. When you touch & hold it will show first the 3, then the 5, you can choose either by releasing at the right moment. To water just one tile, use the usual quick touch. The gold can waters 9 in a 3x3 square, but first shows the 3 in a line, 5 in a line, then 3x3. And the iridium can waters up to 3x6, showing all the lesser options first. It can be tricky to release at the right moment but there is no penalty for pouring too much water everywhere.