Upgrades are used to earn more currency per click/per second. Each upgrade has a specific amount added to your currency for the related action.
If you purchase a green upgrade, this will add 1-3 currency per click. Yellow upgrades will give you 1-2 currency per second. Each new purchase will add the same value each time you purchase it.
Upgrade costs rise after each purchase.

When cost multipliers say “by the amount of,” this is including base cost counting as 1. Meaning, the first purchase of each upgrade marks the multiplier as x2; the second will mark it as x3, and so on.
This is so the multiplier activates a higher cost instead of paying base cost for two purchases.

These upgrades which are purchased with the green buttons are used to earn more currency each time you click the big blue button.

First Green upgrade
Effect: +1 Currency /click
Base cost:
Cost multiplier: Base cost multiplied by the amount of Citizens you currently have

Second Green upgrade
Effect: +3 Currency /click
Base cost:
Cost multiplier: Base cost multiplied by the amount of Businesses you currently have

These upgrades which are purchased with the yellow buttons are used to earn currency each second without having to click the big blue button.

First Yellow upgrade
Effect: +1 Currency /sec
Base cost:
Cost multiplier: Base cost multiplied by the amount of Helping Hands you currently have

Second Yellow upgrade
Effect: +2 Currency /sec
Base cost:
Cost multiplier: Base cost multiplied by the amount of Accountants you currently have