Question about weapon models

Anyone here knows how to change the weapon models? I want to change the level 55 grenade launcher into the model of the relic Thartaros. I tried unpacking the NET55.RFS and and changing the RFT file of the 55 grenade launcher into the RFT of the Thartaros relic i got by unpacking the WET00.RFS. Also changing the files for mesh but when i login it still doesn't work. The model stays the same. I already tried searching both here and in google but no luck for me. If anyone knows how i appreciate it. :):

edit the weapon icon of what weapon do you edit in item.dat
find the icon code the Relic Thartaros then copy the code ,
after that delete the icon code of the grenade launcher then paste the code icon relic Thartaros at there

Thank you so much for helping me. Finally it worked :)