Stop Using a Static Value – How to Get the Drop Down Again

I followed these instructions to set a static value for a field. However, I no longer want to use the static value – I want to change back to using a value from the drop down menu, but there doesn’t seem to be any way to do this.

Can you please provide guidance on how to stop using a static value?

(One workaround I’ve found was to delete the field from the field mapping, then save it, then re-edit the field mapping, insert the field I deleted and then select from the drop down. Is that the intended way to do it?)