HELP NEED TO DOWNGRADE NTT DOCOMO ht-03A FROM 1.6 TO 1.5

[Guest hanrel cellphone]

i have an HTC ntt docomo ht-03a android with the following info:

sapphire EVT1 32B ship s-on G

HBOOT 1.33.007 9SAPPHIRE 2000)

CPLD - 10

RADIO 2.22.19.261

JULY 31 2009

version 1.6 android

I have already an unlock codes to unlock this phone but the problem is when i insert a sim the phone doesnt ask for any unlock code. it opens directly to main menu indicating NO SERVICE>>>>> i have read many issues about this and some had to downgrade my phone from version 1.6 to version 1.5

i dont know how to do this >>> i have already made a goldcard to start with but i dont know what file to use as the next step...can any one here help me or guide me do downgrade my phone coz im a newbie and i dont want to take the risk ...thanks