Quest reset?

[RichPaka]

Can quests be start new again? cause all Bosses are dead but the quest said that i have to finished them?

[Sunflare]

Side quests can be abandoned and retaken, yes. Story quests, however, can't be. You just have to do the quest properly.
You can reset an instance by typing "/reset" (+ Enter) in chat or by opening your profile (P) - instances (bottom right) - reset instances. You'll then be able to kill whatever needs to be killed. Just don't bother too much with quests while levelling up. Clear dungeons and move on instead.