You would need 3 bronze greenhouses to make a silver greenhouse. You would need 3 silver greenhouses to make a gold greenhouse. Once you have the 3 of the same type of greenhouses, you can go to the shop, under buildable, you�ll see an �upgrade� yellow button. All you have to do is press the yellow button and the new upgraded greenhouse will be located in your storage.
Here's a helpful blog post explaining the mechanics of the greenhouses!