Level Max or Promo-Z's Learning attack. The normal playing of a Pokemon LV.X card from your hand is to play it onto your Active Pokemon, as is per the text on the card (see post directly above). If there is an effect of a card (be it an attack, trainer, or whatever) that allows you to play a LV.X card in a specific manner (Level Max and Promo-Z's Learning attack are so far the only means of doing so iirc), then you do what that text tells you.
Under normal circumstances you cannot Level-Up a Pokemon on your Bench. However, if you Level-Up your Active Pokemon and it retreats (or switches), the Level-Up card is retained.