The card text of Level Up!:
Send 1 face-up monster on your side of the field that has "LV" in its card name to the Graveyard to activate this card. Special Summon the monster that is written in the card text of that monster from your hand or Deck, ignoring the Summoning conditions.
The text of Horus LV6:
As long as this card remains face-up on your side of the field, it is unaffected by any Spell Cards. During the End Phase of a turn that this card destroyed a monster as a result of battle, send this card to the Graveyard to Special Summon 1 "Horus the Black Flame Dragon LV8" from your hand or Deck.
I'm not sure what your last question is. If by "tribe" you mean "tribute", then feel free to tribute Horus LV6 to normal summon/set a monster. Level Up! will not replace the tribute required to summon higher level monsters.
I hope this helps.