// Vertices
float vertices[] = {
  -0.5f, -0.5f, 0.0f,
   0.5f, -0.5f, 0.0f,
   0.5f, 0.5f, 0.0f,
  -0.5f, 0.5f, 0.0f
}

// Indices
unsigned int indices[] = {
  0, 1, 3,
  1, 2, 3
}

// Edges
unsigned int edges[] = {
  0, 1,
  1, 2,
  2, 3,
  3, 0
}

// Normally prepare vao,vbo,ebo for vertices and indices.
GLuint vao, vbo, ebo;
// And then another pair for edges.
GLuint edgeVao, edgeVbo, edgeEgo;

glGenBuffers(1, &edgeEbo);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, edgeEbo);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(edges), edges, GL_STATIC_DRAW);

// And then rendering
// First pass render normally
glBindVertexArray(vao);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
glBindVertexArray(0);

// Second pass, render edges
glBindVertexArray(edgeVao);
glDrawElements(GL_LINES, 8, GL_UNSIGNED_INT, 0);
glBindVertexArray(0);