As others have said: Improve your posture. Keep your back straighter. That is pull your belly in. You can do this while cycling, and even at other times, such as while walking. In the beginning, you may have to make a conscious effort to do it. After some time, it may come more naturally. You will find you are using some different muscles, and need to build up the strength of them.
Also abdominal strengthening exercises. You can do these by lying on your back and raising your legs. You can look on the internet for other abdominal exercises.