With the right adjustments you can avoid back, knee or neck complaints. You can raise or lower the bicycle saddle and slide it forwards or backwards across the rails. Besides, you can fix the saddle horizontally or with the point slightly up or down. With the right height your legs will still be a little curved when the peddle is fully down. You should also be able to reach the ground when standing still.
To determine the height of the handlebar you sit down on the saddle and swing your hands forwards. You should have a slight kink in the elbow while you can hold the handles properly and sit upright. At a city bike you usually choose a more upright position whereas at a sporty seat or bike you tend to lean forward. Use the bolt on top of the handlebar to change the height. With the bolt underneath the handlebar you change its horizonal position.
Used tools are a hook wrench 6 and a ratchet wrench or hex 13.