Upgrading is adding materials & stuff to the buildings to make them more beautiful. The 3x3 buildings (farmer's market, opera, stable, etc) start out plain and basic, then when you see the little arrow pointing up and you click on that to activate the growth stages, that is upgrading. The fun thing about playing this game is that there are so many ways one can play it. You can upgrade or choose not to. When I temporarily choose not to, I get more compasses. Even though those buildings generate cash, they take up space and a long time to upgrade, so I choose to get the land first then the buildings later. Everyone plays how they want to. I think its creative. I hope this explanation helps. (-:

One way is to give plant gifts. Giving plants that cost coins but reward with purple diamonds works! my friends in the game helped me reach 799. Also the more you play you will eventually get 20 diamonds in the green and pink boxes. They also come as rewards when completing a scene, so far I've seen 5 diamonds given as a reward.