Insert, Update values in existing custom table and Select values from that table. Magento2

If you have model for your custom table then you do all these stuffs in the below way

Try this,

Di method :

Inject your model in your constructor

 protected function __construct(
 ....
 \Vendor\ModuleName\Model\ModelName $customTable,
 ....
 )
{
   ...
   $this->customTable = $customTable;
   ...
}

then on your execute function from controller

execute()
{
  // insert
  $model = $this->customTable->create();
  // update
  $model = $this->customTable->create();
  $model->load('id',$id_to_update);

  $model->setField_Name('values to be stored');
  $model->save();
}

In order to get collection

$model = $this->customTable->create()->getCollection();
foreach($model as $item){
    echo $item->getFiledName();      
}

For more information Get custom table information

Save values into custom table

Hope this helps.