To append a column to another and update the value using MongoDB you can follow these steps if you are on MongoDB compass

Open the shell command panel on the base of the MongoDB window called

type use <collection>

where <collection> is the name of your database without the angle brackets

the the code below

db.collection.aggregate(
    [
        { "$addFields": { 
            "name": { "$concat": [ "$firstName", " ", "$lastName" ] } 
        }},
        { "$out": <output collection name> }
    ]
)

UPDATE A COLUMN