change it in the table of contents by double-clicking on the layer, then in properties, edit how it displays... there is no magic bullet for this one beyond coding to edit the legend itself http://desktop.arcgis.com/en/arcmap/latest/analyze/arcpy-mapping/legendelement-class.htm
To be clear, while Dan is correct about having to change a layer in the table of contents, the part about coding to edit the legend for layers is not correct as far as I can tell. Legend items(i.e. layers) are read-only so you can not change them. You would have to code to edit for the table of contents, not the legend. Coding for the legend and table of contents require different methods in ArcPy.