How to change the dimension of the video that I am creating
You can't change the dimension of a video while creating it. You need to first export the video. If the template in which the video has been made is available in more dimensions, you can duplicate it in those dimensions.
For a detailed tutorial on duplicating videos in another dimension, see How do I create the same video in different dimensions?
Click here for a short video to take you through the process. Or click on the thumbnail below: