All upgrades are available in the My Kitchen tab in the bottom left corner of the game's main screen. There, you can find a list of all available upgrades for the restaurant and information on how to unlock a new one. 

There are 3 types of upgrades:

▪ Increase portion size;
▪ Speed up cooking time;
▪ Increase dish price.

To upgrade your kitchen, you need to have enough currency. The better the upgrade, the higher the cost.

Each upgrade grants a number of stars that go toward the progress bar for unlocking the next restaurant.