Update query in MongoDB shell

This is because in second parameter of update function you need to use $set operator to update location_country as in example below:

db.checkin_4e95ae0926abe9ad28000001.update(
   {location_city:"New York"}, //find criteria
   // this row contains fix with $set oper
   { $set : { location_country: "FUDGE!"}}); 

Here you can find a list of available update operators.

Changed in version 3.6. Following is the syntax for update :

db.collection.update(
   <query>,
   <update>,
   {
     upsert: <boolean>,
     multi: <boolean>,
     writeConcern: <document>,
     collation: <document>,
     arrayFilters: [ <filterdocument1>, ... ]
   }
)

Example :

db.getCollection('products').update({},{$unset: {translate:1, qordoba_translation_version:1}}, {multi: true})

In your example :

db.checkin_4e95ae0926abe9ad28000001.update(
   {location_city:"New York"}, //query
   // $update query
   { $set : { location_country: "FUDGE!"}});

By default, the update() method updates a single document. Set the Multi Parameter to update all documents that match the query criteria.

Example 2 :

   db.checkin_4e95ae0926abe9ad28000001.update(
       {location_city:"New York"}, //query
       // $update query
       { $set : { location_country: "FUDGE!"}}, {multi: true});

db.m_country.update(
    {"countryId": "962a0935-bf3d-4f63-a53c-254760273ede"}, 
    {$set: {'countryPopulation': '12540000'}})