How can i downgrade my psp 3004 wiyh a firmware of 6.20

You dont need to downgrade since you can use things like 6.20 TN-C/D or 6.35 PRO-B,but if you want to downgrade you must know if your PSP has a 03g or a 04g module.03g can downgrade to 5.00 then use ChickHEN R2 and then 5.03 GEN-C.04g modules cannot downgrade below 6.00 but there is no point in doing that since there is NOTHING avilable for 6.00/6.10.You can find out if you PSP is 03g or 04g with PSPIdent (you need HEN ofcourse).

!!REMEMBER!!
DOWNGRADING is AWLAYS risky especialy when you dont have a Pandora capble PSP.Downgrade at your own risk remember that there is no unbrick for 3000/GO/v3 motherboards.