If the Federal Reserve (Fed) wants to increase the money supply, it can purchase Treasury securities. This process is called "open market operations." When the Fed buys Treasury securities, it pays for them with money that it creates out of thin air. This increases the total amount of money in circulation, which can help to stimulate economic growth.
On the other hand, if the Fed wants to decrease the money supply, it can sell Treasury securities. This process removes money from circulation and can help to slow down economic growth or combat inflation.