Soma is ToH´s main equipment in the game, it replaces weapon AND armor, too.
The only thing you can change in the game are accesories.
Since it cant be changed, you can customize it with
Soma Build.
Opening the menu, and opening the equipment section (marked with Shing´s Soma graphic), you have three options, that are Soma Build, Soma Evolve and Accesories.
After accesing Soma build you will have a load of things you can choose from, some which are greyed, some which arent. Those are the things you can improve, in which you will waste materials gained from battle to learn those skills.
There are:
Action Skills (pair of green boots) which will teach you special actions in battle like double jump, aiming step, backstep, manual mode.
Parameter Skills (muscle arm) which improve various status.
Support Skills (light blue little symbol) which makes you get various effects, many which are recurrent in Team Destiny games (like heal yourself a bit when you heal someone else with a spell)
Battle Skills (purple little symbol) which will teach you lovable artes
When you open a skill to improve you are issues with a window which lists the items needed for upgrading and the items you have.
After you have made enough upgrades you will be able to improve your soma through
Soma Evolve.
There are three options:
Red: Concentrates on Parameter Skills and Support Skills
Blue: Concentrates on Paramater Skills and Battle Skills
Green: Concentrates on Support Skills and Battle Skills
Action skills are gained equally on all three evolution categories

Apart from the skill bonuses, you will gain more status for your Soma (mainly a lot more EG and CG!)
---How to get the materials??----To get materials for getting your Soma skills, you have 2 ways.
Either you buy them, or you get them at dungeons. The most common way is battling, but also there are material spots. When in a dungeon, you will see in the lower screen sometimes a rainbow color mark, that is a spot where you can get materials, which respawn and gives random materials (sorta like ToA search spots)
If you cant understand what to buy, its as simple as buying one single unit, and checking out if in the soma build one of the materials missing increased in one.

If it isnt, keep it in any case, it might be used later