You can override Default Pawns per Level in two ways:
But like Ninjin said if you just want to change some Meshes and Textures around. You can drop your Character into the Level and adjsut what you need (make sure Auto posses by Player 0 in the Details Panel)
Or use something like your LevelBluprints Begin Play Event GetPlayerCharacter->CastToYourChar->Adjust what you need via Script.