The game provides several ways for your City to earn funds that you can use for upgrades. Moreover, you will need to save enough resources to pay for the cost of the update. It can take a while, especially at the start. The game also offers in-app purchases. You can choose to spend real money and purchase the resources you need. Also, it will speed up the process and allow you to start upgrading.
Play the latest and newest versions of Plants Vs Zombies Game in one convenient place.