1. 

u16  g_summ  = 0;

u8 buf[2] = {0x10, 0x05};

g_summ = (buf[1] << 8)  |  buf[0];    //  g_summ = 0x0510

2. 

u16  g_summ = 0x0510;

buf[0] = (u8 )g_summ;             // buf[0] = 0x10

buf[1] = (u8 )(g_summ >> 8);  // buf[1] = 0x05