All the useful gold-producing buildings can be upgraded. Unfortunately, you can't merge them, but you can use gold to increase their level and make them more profitable! To level up the building - press the green button with a + on it.

The number on the button shows how many levels you're able to afford with your current gold supply. The upgrading cost is also shown on the button. Using the blue bar above, you can estimate how close you are to the next big income boost. The light blue part of the bar shows the progress you'll get after upgrading. The yellow part shows your current progress.

Plan your budget wisely: upgrading the building for a few levels might not give you the increase you want. When you reach an income boost milestone with your building, you get a small number of bonus elixirs.

Another way to permanently boost the building's profit is by researching more advanced technologies using cards.