If you click in the arrow by your Bump Mapping channel, you will get taken to your file node, then click on the forward arrow beside the file textbox, then you will get taken to your bump2d node. There you can adjust the Bump Depth. (You can also access the node in the hypershade, but this is fast, although there's probably a faster way).

I often end up using something like 0.2 for the bump depth otherwise it's so extreme, but of course it all depends on your project.