I have a video file but videoReader doesn't allow frame rate change. I think i have to rewrite each of the frames in a loop to change the rate but don't know how to do that. if anyone has any suggestions thanks in advance
You are fully right. First read all the frames using the read command for VideoReader. Then make a VideoWriter object and use the FrameRate property to set the frame rate you need and then write the video file using writeVideo. If you read the documentation and follow the examples given there it is quite easy to do this.