I keep my olive during winter inside and temperature is around 13-20 °C and
stands next to window...so has enough light but its leaves keep falling off so
how to stop it?

You are keeping it too warm. Olives need a cool rest in the winter.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)