For switching main characters, you'd have to have min. lv60 I believe. Then click on the "character" icon. Immediately under your character's large portrait is a *on "change", click on that and you'll see a list of slots (like the picture shown earlier on pg2). Empty slots means that you've not unlocked that character.
To unlock a main: simply select them when clicking on a blank slot, you'll enter an easy match with them and after the victory, the main is available for selection. You can open more slots, to a max of 5 total by lv78. (Max slots are gated by level).
Once you change to a new main, a timer will start for 24hrs, during this time if you wish to change again, it's 100ingots. Once the 24hr timer expires, changing main is free again. And so the cycle continues...