Answer by
LeheckaG (1826)
The simplest short-term/temporary fix is to exterior caulk above & around the window frame.
Longer-term fixes are either to:
Remove the window frame and see if it was installed properly to manufacturer's specifications
(in recent housing boom/busts, many window frames were improperly installed)
Install an awning above the window frame so that it catches/diverts the rain/weather.