Help me understand the script.

While creating a map, I had an idea. When capturing, for example, a sawmill, a message would appear stating that the sawmill could be upgraded by spending a certain amount of money. Okay, that's not difficult to implement. The question is: how can I make it so that after upgrading the sawmill, it produces more wood each day (e.g., +2)?
If this is not possible, can it be done in some other way?