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CountryData[ "UnitedStates", {"Population", 2014} ]

322 422 965 people

CountryData[ "UnitedStates", {"Population", 2015} ]

Missing[ "NotAvailable" ]

How can I update it?
I am new to Mathematica.

CountryData does appear to be out of date, and that is a shame. Many of the XXXData functions now act as wrappers to call EntityValue. Run TracePrint[PlanetData["Venus", "AngularDiameterFromEarth"],_EntityValue] to see that this is true.

But CountryData has not been updated to work this way. So we can call EntityValue directly for the data. To find out what the right syntax would be I will use Ctrl=:

becomes

which is a formatted form of

Entity["Country", "UnitedStates"][

    EntityProperty["Country", "Population", {"Date" -> DateObject[{1978}]}]

] 

I can query for a date range as well. Using Ctrl= on "US population 2000 through 2018" returns this input expression:

Entity["Country", "UnitedStates"][

    EntityProperty["Country",

        "Population",

        {"Date" -> Interval[{DateObject[{2000}], DateObject @ {2018}}]}

    ]

] 

which returns a TimeSeries object suitable for further computation or visualization.

You should report to WRI. As a workaround you may use the entity functions instead; it is a bit verbose.

First you can get a list of qualifier values of an EntityProperty by

EntityValue[EntityProperty["Country", "Population"], "QualifierValues"]

{Age->{Adult,MiddleAge,PreSchool,SchoolAge,Senior,Young,YoungAdult},

 CitizenshipStatus->{BornInPuertoRico,BornInUS,BornToAmericanParents,NaturalizedCitizen,NotCitizen,TotalCitizens},

 Date->{},

 Gender->{Female,Male},

 HispanicOrigin->{Argentinean,Bolivian,CentralAmerican,Chilean,Colombian,CostaRican,Cuban,Dominican,Ecuadorian,Guatemalan,Hispanic,HispanicOrLatinoAllOther,Honduran,Mexican,Nicaraguan,NotHispanic,OtherCentralAmerican,OtherHispanicOrLatino,OtherSouthAmerican,Panamanian,Paraguayan,Peruvian,PuertoRican,Salvadoran,SouthAmerican,Spaniard,Spanish,SpanishAmerican,Uruguayan,Venezuelan},

 MarginOfError->{MarginOfError,StandardError},

 Percent->{Main},

 Race->{AmericanIndian,Asian,Black,NativeHawaiian,Other,TwoOrMore,White,{All,Hispanic}},

 TwoOrMore->{ThreeOrMore,TwoIncludingOther},

 UrbanRural->{Rural,Urban}}

"Date" is a qualifier. This post gives details on how to specify a "Date" qualifier (156014); an area where the docs can improve.

EntityValue[

 Entity["Country", "UnitedStates"],

 EntityProperty["Country", "Population", {"Date" -> DateObject[{2015}]}]

 ]

319,929,162 people

However, you do have a bit more flexibility with this syntax.

EntityValue[

 Entity["Country", "UnitedStates"],

 EntityProperty["Country", 

  "Population", {"Date" -> Interval[{DateObject[{2015}], DateObject[{2019}]}]}]

 ]

and

EntityValue[

 Entity["Country", "UnitedStates"],

 EntityProperty["Country", "Population", {"Date" -> All}]

 ]

Hope this helps.