Double-click the chart. Microsoft Graph will open to help you modify the chart.
The data which produces the chart is in one window. You can change labels safely, but if you change the data that actually produces the chart, the chart will no longer accurately reflect your database.
To change the formatting of chart elements (e.g. column color, axis scaling, labels), simply double-click the chart element you wish to change. The Format dialog that appears will help you make your desired changes.