Mongo update query equvalent to SQL

Question

I need equivalent mongo query to below SQL query:

Update tableName set tableName.col1 = tableName.col2

Answer 1

Update: I had given incorrect solution earlier. As the answer is accepted, updating it with the right way

db.collectionName.find().snapshot().forEach(function (elem) {
        db.collectionName.update({_id: elem._id}, {$set: {col2: elem.col1}});
    }
);

Use normal update query along wiht $set to set new field value

db.collectionName.update({}, {$set: {col2: '$col1'}}, {multi: true})