After you have bought a new business, you can start to upgrade it with cash. Each upgrade increases the earn rate of that business. The price of the upgrade will increase a bit after each upgrade. Once you have done a certain number of upgrades, the business becomes ready for the level up. Below the earn rate indicator of the business is an indicator that shows the how many upgrades you have already done and the number of upgrades needed for the next level up. (e.g. “71/100”).
If the upgrade price is shown in red colour, it means that you do not have enough cash. In this case you can use gold bars to buy missing cash. Just tap “Buy” button and a pop-up with buy with gold bars option is shown. Alternately, you can wait until your businesses have generated enough cash for the upgrade.
After you have reached the upgrade limit, the business can be levelled up. To do that you need to have enough free customers (shown on top left corner of the screen). Level up is always a big improvement for the earn rate. After the level up, new upgrade target will be available for the business.
If the number of required customers is shown in red colour, you do not have enough customers for the level up. Then your only option is to go and try to spin more as customers cannot be bought with cash or gold bars.