How to add a new data field and data to an existing list

This is a simple task I’m sure but I haven’t been able to track down the method from the online documentation.

I have created a list in the Directory with contact details and some other fields.  (I have since run a survey and populated the list with some of the survey responses) I now need to add an additional field, with (external) data from a spreadsheet, to the existing list (before doing another survey)

What’s the best way of achieving this?

Thanks in advance