Let’s pretend we have a cookbook named “wonka”

in ~/chef-repo/cookbooks/wonka/attributes/default.rb

default[:wonka][:gobstopper]  = "everlasting"
default[:wonka][:kazoozle]    = "alittle"

We want to override the value of [:wonka][:gobstopper] for a node named “oompaloompa01”

Without an override defined your json should look something like the following
You can issue:
$ knife node edit ooompaloopma01

{
  "run_list": [
    "recipe[chocolate_factory::default]"
  ],
  "chef_environment": "development",
  "name": "oompaloompa01",
  "normal": {
    "chef_environment": "development",
    "name": "oompaloopma01",
    "normal": {
      "tags": [

      ]
    },
    "tags": [

    ]
  }
}

Now let’s say you want to define default[:wonka][:gobstopper] = “allgone” with an override
Your json should look like the following

{
  "run_list": [
    "recipe[chocolate_factory::default]"
  ],
  "chef_environment": "development",
  "name": "oompaloompa01",
  "normal": {
    "chef_environment": "development",
    "name": "oompaloopma01",
    "wonka": {
      "gobstopper": [
        "allgone"
      ]
    },
    "normal": {
      "tags": [

      ]
    },
    "tags": [

    ]
  }
}

that should commit the override to that specific node.
you can check the values of a node’s attributes with the following command ( where the attribute group you want to look at is “wonka” )
$ knife node show oompaloompa01 -a wonka

Now just run chef-client on oompaloopa01 and you should see the override take effect.