How to divide one triangle in mesh?

Do you mean you want to subdivide the mesh?

As @cherno says, you have to reset the vertices and triangles arrays. Depending on what you are doing, you could simplify things by hacking a bit. That is you collapse the large triangle by giving it three of the same vertices, then add the new vertex and two triangles to the end of the two arrays.

ok. i'll do that way. I was just searching if there was other simple way.)