In rough terms, there are only two ways to make money. Trade your time for money or trade your capital/assets for money. Most people does both throughout their lives. They have a job where they trade their time for money and they have some assets in form of surplus capital in a savings account or in investments that generate some money.
With this in mind, there are only four ways to increase your income. On the “time for money” side, you can spend more time working or you can learn unique skills so you can demand a higher salary for your time. On the “asset for money” side, you can acquire more assets or use the ones you have more efficiently.