How do you convert video to 16:9 or keep the original aspect ratio?

The output video's aspect ratio is decided by the aspect ratio of the original video or the output settings you've made. 

To convert video to 16:9 or in original aspect ratio, there are 2 situations:

If the original aspect ratio is 16:9, simply click the Advanced button at bottom, choose an output format from Profile > Common Video and choose Keep Original from the Resolution drop down list.

If the original aspect ratio is not 16:9 and you need to convert the video to 16:9 video, you have to crop the video, see how below:

1. Import the video to the software and click the Advanced button to choose a format from Profile > Common Video and set the resolution to any 16:9 resolutions, like 1280*720.
2. Go to the menu bar Edit > Video Edit > Crop. In the crop panel, set Zoom to 16:9. If there is black bar in the right preview window, drag the handles in the left window to cut out the black bar from the source video. You'll see the immediate changes in the right preview window.

Note:
If the source video is DRM protected and you want to keep the original aspect ratio after conversion, you need to make settings as this: go to Tools > Preferences and in the DRM tab, set Capture Size as Keep Original or Half of original. To convert the DRM protected video to 16:9, you need to convert the DRM protected video to DRM-free video first, and then load the converted video for cropping as described above.