Hello,
I want to install video transmitter on my hexacopter but I have one question. Mounting antenna of video transmitter upside down will be better in transmission and in range too? Or just mounting normal will be good, antenna pointing upwards? What is better?

If it is a cloverleaf antenna then the radiation pattern is like a doughnut with a null right above and below the antenna. So mounting it right way up or upside down shouldn't make much of a difference.