First you will need packfile manager, after that it's relatively easy to do.
To play around with unit armor, helmets, cloaks etc you need to open up the "models_rome2.pack".
1. Open "models_rome2.pack" with packfile manager.
2. Expand the "variantmeshes" entry.
3. Expand the "variantmeshdefinitions" entry.
4. Locate the unit you want to play around with, for example "roman_principes.variantmeshdefinition"
5. Right click on "roman_principes.variantmeshdefinition"
6. Choose "open" then "open as text".
7. The information you can now edit will be in the righthand window in packfile manager.
I find it easier to play around with these files when I have 2 packfile managers open. I will find a unit with a helmet or whatever I like, open that unit in one packfile manager then in the other edit the unit I want changed.
Changing weapons is a bit different as you will also need to edit "data_rome2.pack" / "DB" / "land_units_tables" / "land_units" / the tables "class", "man_animation", "primary_melee_weapon", and or "primary_missile_weapon".
Other than that have some fun, oh it's always a good idea to back up any pack files you want to play around with, I throw mine in a folder called "Rome2 originals", that way if the game crashes due to a change i've made I can just copy and paste the original file back into the Rome 2 data folder and everything is back to normal
