How Do I Update A Record Using Entity Framework 5

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C#5.0

i am using entity frame work 5 database first workflow, just my first attempt, i am trying to update a record attribute, but it doesn't seem to work, the error message is:

the property 'device_name' is part of the object's key information and cannot be modified

here is my code:

device dev = dbEntities.devices.First( i => i.id==1 );
                dev.device_name="newUpdateValue";

                dbEntities.SaveChanges( );

also tried this:

protected int UpdateMethod(int Id, string newValue,string modelNumber)
       {
           using (var ctx = new GSC_NewSchoolCoreEntities( ) )
           {
               var query = ( from q in ctx.devices
                             where q.id  == Id
                             select q ).First( );
                   query.device_name  = newValue;
                   query.model_number= modelNumber;
                   int result = ctx.SaveChanges( );
                   return result;
           }
       }

the calling method:

UpdateMethod( DeviceID, DeviceName, ModelNumber );

still having the same error message, i have search online to see if i could fix this myself, but none of the online solutions work for me! device_name and model_number are what i want to update but not the id, Please is there anything i am missing? Thanks for your contributions.