You can override the default icons by applying custom style. In order to override the 'down' arrow and replace it with 'left' arrow, you can use the style below:
Note, that the above will replace all arrows on the page. In case you need to replace only the arrows in specific PanelBar, you can add its id to the selector. For example: #panelbar .k-i-arrow-60-down:before
I hope that the linked Dojo
example will be helpful.