To: pritam
Dear user,
Unfortunately it is not possible to change the aspect ratio only on the part of the video.
You can try to use AVS Video Converter to convert the part of the video where you need to change the aspect and delete the other part:
http://onlinehelp.avs4you.com/AVS-Video-Converter/Features/Editing/Timeline/DeletingScenes.aspx
To avoid loss of quality please use the input file bitrate, frame rate and frame size for the output.
You can select the output file frame size in AVS Video Editor when saving the video by pressing on the Advanced button (see the screenshot attached).
The input file aspect ration can be changed pressing the Edit button on the top toolbar of AVS Video Editor - Settings - Aspect ratio.
Please let me know if I can be of any further assistance to you.
Kind regards