How to add and clean out property values

What I’m trying to do

I’m trying to add more people to the property ‘person’ and clean that out a bit. The property is a list type.
Cleaning : In the past I would write for example ‘John and Josh’. I want to make this ‘John’ and ‘Josh’.
Adding : Also when I meet with new people, I need to add a new name to the list. I dont seem to be able to pull this off…

Things I have tried

using the Side pannel of the properties, Commands on properties, Righty clicking etc… Commands on properties