Hello. My name is Mike. I'd like to try and assist you with your issue. Is this for a computer monitor or a TV screen?
Yes. I understand. But if the screen is connected to your computer then you simply need to right click anywhere on the desktop and go to the display settings to change it.
The aspect ration you are referring to is your screen resolution. Follow the instructions I provided and you choose different screen resolutions the most commonly used are 1920X1080 or 1600x1200.
I provided you information on how to change your screen resolution. You did not state that you were using AutoCAD in the beginning, and AutoCAD is not in my area of expertise. I will opt out and allow another expert to try and assist you.