copy the original save onto an SD card
put it in a safe location on your computer
copy the downloaded save onto the SD card
delete the original save off the wii.
copy the d/led save onto your wii.
play said game.
as was said you can't copy wifi games
also, you can't have two save files on the wii at the same time.
Wii Games: Legend of Zelda: Twilight Princess, Heatseeker, Excite Truck, Wii Play, NHL 2k9, Super Mario Galaxy, Super Strikers Charged, Shaun White Snowboarding, The Bigs, Guitar Hero: World Tour, Ghost Squad, Call of Duty: World at War, Wii Sports Resort, Punch-out!
VC: Wave Race 64