asm/dissasm with nasm

Hi,
I need to modify many code of a exe file, doing it with ida, ollydbg... is really tedious, i readed somewhere that with nasm you can dissasemble a exe, rewrite, and assemble the code again.
But i cant find the way to do it.
Somebody know how i can do it?
[280 byte] By [MazarD] at [2007-11-20 8:23:54]