Tools needed:
MyWarCraftStudio [Download v0.9]
Start by clicking on the folder icon
Select common.mpq and open it
go to ITEM -> ObjectComponents -> HEAD
RaidWarrior_A_01 (located in common.mpq) -> Warrior T1
RaidWarrior_B_01 (located in common.mpq) -> Warrior T2
RaidWarrior_C_01 (located in common.mpq) -> Warrior T3
RaidWarrior_D_01 (located in common.mpq) -> Warrior T4 = Warrior Arena S1
RaidWarrior_E_01 (located in patch.mpq) -> Warrior T5 = Warrior Arena S2
RaidWarrior_F_01 (located in patch.mpq) -> Warrior T6
RaidPriest_A_01 (located in common.mpq) -> Priest T1
RaidPriest_B_01 (located in common.mpq) -> Priest T2
RaidPriest_C_01 (located in common.mpq) -> Priest T3
RaidPriest_D_01 (located in common.mpq) -> Priest T4 = Priest Arena S1
RaidPriest_E_01 (located in patch.mpq) -> Priest T5 = Priest Arena S2
RaidPriest_F_01 (located in patch.mpq) -> Priest T6
RaidHunter_A_01 (located in common.mpq) -> Hunter T1
RaidHunter_B_01 (located in common.mpq) -> Hunter T2
RaidHunter_C_01 (located in common.mpq) -> Hunter T3
RaidHunter_D_01 (located in common.mpq) -> Hunter T4 = Hunter Arena S1
RaidHunter_E_01 (located in patch.mpq) -> Hunter T5 = Hunter Arena S2
RaidHunter_F_01 (located in patch.mpq) -> Hunter T6
etc..
BeM -> Blood Elf Male
BeF -> Blood Elf Female
DrM -> Draenei Male
DrF -> Draenei Female
DwM -> Dwarf Male
DwF -> Dwarf Female
GnM -> Gnome Male
GnF -> Gnome Female
HuM -> Human Male
HuF -> Human Female
NiM -> Night Elf Male
NiF -> Night Elf Female
OrM -> Orc Male
OrF -> Orc Female
ScM -> Undead Male
ScF -> Undead Female
TaM -> Tauren Male
TaF -> Tauren Female
TrM -> Troll Male
TrF -> Troll Female
Scroll down to the helmet you want to fix and select your race (I take Human Male for example)
Right click -> Copy Treepath
Select the race you modelchanged into (I take Orc Male for example)
Right click -> Extract File To...
Save it
Pack -> Create MPQ Archive
Are you sure to close current archive -> ok
name the new MPQ: patch-#.mpq (# = a-z 3-9) (for example: patch-a.mpq)
Save it
Pack -> Add file to archive...
Click on the [...]
Select the file you extracted and open it
Mark the text and press CTRL + V (or rightclick -> paste)
ok
It should look like this now
Pack -> Save and Close Archive
If you can't find your helmet in common.mpq look for it in patch.mpq !