For the tech-tree building? What do you mean? Every race has a "Build" ability. "Build-Human", "Build-Orc", etc. They are unique based on the unique abilities each race has for building. Humans can have multiple peasants "repair" a structure to build it faster. Undead can summon buildings, etc.
The actual structures that are available by that build ability are controlled by the "Structures Built" field on the UNIT who has the build ability. So if you look at the Peasant, he has the "Build-Human" ability, and if you check out his "Structures Built" field, it has all of the stuff he can make (Farm, Town Hall, Blacksmith, etc).
If you want to change the icon, first figure out which build skill you want, and just change the icon of it in the Abilities tab. Then give that custom build ability to your unit, and add the structures you want that build ability to have.
Also, if you want to control where on the grid the various structures appear in the build menu, then you have to go under units to the structures themselves, and change their "Art - Button Position (X)" and "Art - Button Position (Y)", with X = 0, Y = 0 being the top left corner.
Hope that helps!