I kept searching online the other day, and figured out what I need is actually cropping videos which including reframing videos at a certain aspect ratio and at a freeform. This article rounded up lots of video cropping software, and I tried some free and lightweight tools on the list. They all worked quite well for me.
To change a project's aspect ratio:
Click to select a project in the Project Library, or open a project so that it's showing in the Project browser, and then choose File > Project Properties. ...
Choose an option from the Aspect Ratio pop-up menu.