All of the buildings that generate cash can be upgraded over time. You cannot merge buildings, but you can spend cash to raise their level and increase their profitability!

To upgrade a building, tap the green + button on its panel.

The number displayed on the button shows how many levels you can afford with your current cash. The cost for those upgrades is also shown there. Above this button, you’ll see a progress bar: the light grey section represents the boost you’ll gain after upgrading, and the yellow section shows your current progress toward the next big increase.

Plan your spending wisely — upgrading just a few levels might not give you the jump in earnings you expect. When you hit an important income milestone for a building, you’ll receive a small amount of bonus blueprints needed to upgrade your permanent boosters.

Another way to permanently increase a building’s profit is by researching more advanced technologies using your cards at the card section. The tenants will significantly increase your earnings and automate them at a certain level. 

Sometimes you’ll spend blueprints to upgrade at your apartments, but you might not see automation unlocked immediately — this just means the building’s card has increased in level, but it still needs more upgrades before it fully automates.