The video aspect ratio is to be selected on the screen where you select the template.
1. Go to the template you wish to use and click on Select.
2. You will then be prompted to select the aspect ratio.
3. Select the Aspect Ratio and click on Continue.
* To change the aspect ratio at a later stage, you can always follow the below path:
Format > Aspect Ratio
Below is a screenshot of the 16:9 format.