When you select an error bar, any changes you make affect all of the error bars in its associated data series.
Click the data series that has the error bars you want to change.
On the Format menu, click Selected Data Series.
Click the Y Error Bars tab or the X Error Bars tab, depending on the type of error bars you want to change.
To select a different type of error bar, click the type you want under Display.
To change the method used to determine the error, select the method you want under Error amount.