Jetting setttings on my yz250f?

i have a yz250f 2002 that wont stop back fireing. the engine is completly standard. i think the jetting is a little lean. i dont have a clue a bout the new 4 strokes. i mean i know every thing there is to know about two strokes but 4 stokes. could you please tell me how to change the jetting, what to change it to and how long it will take. thanks