Change default audio track of Video file using ffmpeg
ffmpeg -i "input.mkv" \
-map 0:0 \
-map 0:2 \
-map 0:1 \
-disposition:a:0 default \
-disposition:a:1 none \
-c copy "output.mkv"



input
use video stream (0:0)
make 2nd audio stream (Hindi) the 1st
make 1st audio stream (English) the 2nd
use the new 1st audio stream (Hindi) as default
remove the default from the new 2nd audio stream (English)
create output without transcoding