The documentation for these two API calls is identical. The latter should demonstrate how to specify which object is to be updated.
// https://backendless.com/docs/js/data_single_object_create.html
Backendless.Data.of( "TABLE-NAME" ).save( { propertyA:"", propertyB:""} )
.then( function( savedObject ) {
})
.catch( function( error ) {
});
// https://backendless.com/docs/js/data_single_object_update.html
Backendless.Data.of( "TABLE-NAME" ).save( { propertyA:"", propertyB:""} )
.then( function( savedObject ) {
})
.catch( function( error ) {
});