you need to create a new folder in

C:\program files\Activision\Rome - Total War\Data\models_unit

called textures and put the new texture in there. You will also need the gloss file so look for

UNIT_EASTERN_HEAVY_INFANTRY_GLOSS.TGA

in

C:\program files\Activision\RomeTotalWar\Data\packs\DATA\MODELS_UNIT\TEXTURES

and put that in your new textures folder. Rename it so it fits whatever you called your new texture eg. if you called it indian_unit.tga rename the gloss file to indian_unit_gloss.tga

Then go to
C:\program files\Activision\Rome - Total War\Data

and go into the text file

descr_model_battle

scroll about 2 thirds of the way down and find the entry for eastern heavy infantry, it will say

type east_heavy_infantry
skeleton fs_spearman, fs_swordsman ; combat spear
indiv_range 40
texture armenia, data/models_unit/textures/unit_eastern_heavy_infantry_armenia.tga
model_flexi data/models_unit/unit_eastern_heavy_infantry_high.cas, 15
model_flexi data/models_unit/unit_eastern_heavy_infantry_med.cas, 30
model_flexi data/models_unit/unit_eastern_heavy_infantry_low.cas, 40
model_flexi data/models_unit/unit_eastern_heavy_infantry_lowest.cas, max
model_sprite armenia, 60.0, data/sprites/armenia_east_heavy_infantry_sprite.spr
model_tri 400, 0.5f, 0.5f, 0.5f

you need to change the texture for armenia so replace that filename with your one
type east_heavy_infantry
skeleton fs_spearman, fs_swordsman ; combat spear
indiv_range 40
texture armenia, data/models_unit/textures/indian_unit.tga
model_flexi data/models_unit/unit_eastern_heavy_infantry_high.cas, 15
model_flexi data/models_unit/unit_eastern_heavy_infantry_med.cas, 30
model_flexi data/models_unit/unit_eastern_heavy_infantry_low.cas, 40
model_flexi data/models_unit/unit_eastern_heavy_infantry_lowest.cas, max
model_sprite armenia, 60.0, data/sprites/armenia_east_heavy_infantry_sprite.spr
model_tri 400, 0.5f, 0.5f, 0.5f

and your done. I am sure there are guides for this on every totalwar forum