Update! To replace more than one file at a time, take a look at Pikazapper’s guide to replacing the entire disc’s contents here.
If you’re loading NSMBw from backups, you can use WiiScrubber to replace files on the game’s ISO and create a custom IOS, eg replacing ‘Mario.arc’ in the objects folder with a ‘Mario.arc’ downloaded from Mario-mods, or the ’01-02.arc’ level file with another downloaded from the NSMB File Depot. You can then run your custom NSMBw ISO with a USB loader or Dolphin (a wii emulator).
If you can’t find where the file you want to replace is on your ISO, check out this list of the NSMBw disc contents. You can, for example, search for ‘Mario.arc’ (it’s the second one, in the ‘Objects’ folder).

To get an ISO for NSMBw, use WiiDump or a similar program to rip a NSMBw ISO from the disc to an SD card or USB device.
when i replace it how can i put it to my SD card??
You either use an SD card, a retail disc and Riivolution, or you replace the files of an ISO, and use an emulator, backup loader or burnt disc.
then there is no way I can play it without having to burn a disc or an emulator on the PC here ????????
lolnoobs
everybody's a noob sometime…
is it possible to make luigi the main character?
There is a way to play as Luigi in one player with cheat codes. It doesn't actually make Luigi the main character, but it lets you play as him just as you would in multiplayer. That's the only way I know of.