If you have reached this point, it means that there is just one thing missing from your game. However, this is an optional section, and it is only for those who have done the optional section of the previous chapter, since we are going to finish what we started.
In the previous chapter, we created a TowerMenuScript that interacts with one instance of the TowerScript stored inside the currentTower variable. In particular, it has access to one variable of the tower called upgradeLevel; however, we have never created this variable. So, open the TowerScript and let's add this one line:
public int upgradeLevel = 0;
It starts with the value 0, since now we have set the TowerMenuScript, the levels of our towers start from 0 up to 2.
Furthermore, TowerMenuScript calls the Upgrade() function of our tower, that we don't have. In this function, we need to increase the stats of the tower, including the level, and then we can also close the TowerMenuScript...