How do I change aspect ratio Windows 10?

That does not answer CaptGreg's question. This is an aspect ratio issue, not a resolution issue. I m having the same problem, using a flat screen monitor with my Acer laptop (the laptop monitor is broken so I have to use the other screen).

Microsoft as usual issued a new version of Windows before fixing lots of obvious problems. They didn't even bother to upgrade their help manual  - in the Windows 10 Help area they are still telling one to go to Windows Media Center to fix the aspect ratio, but there is no more Windows Media Center in Windows 10!

HOW DOES ONE CHANGE THE ASPECT RATIO IN WINDOWS 10? 

You would do that either with the Management Control Software that came with your Graphics Card or with your Monitor's own controls, not in the Windows OS, although you should be able to access any control software through Device Manager and Graphics Properties.

So, check your Monitor manual and what control software is available for your Graphics Card.

Device Manager says that the monitor is working properly, driver is up to date, etc.  This didn't work. It is clearly a Windows 10 problem as the aspect ratio was fine before installing Windows 10.

I have heard that you may have to install a W10 compatible driver to your graphics card.  I am using a NIVIDIA GE Force driver, and have found a way to reset the aspect rario using the NIVIDIA Control Panel to 1920x1080.  If you do your arithmetic, you will find that 1920 divided by 1080 = 1.777777 which is the same as 16x9.

I have only just found this out. In the meantime, I got fed up with W10 and went back to W7, which I am familiar with and which will be around for some time.  Everything works again.

Somewhere today whilst trying to come to terms with Win 10 doing such useful things as rebooting after a weird error message (at least six times), giving me a blank screen before logon, freezing with a tiny windows symbol on screen, etc, I came across a comment from some lovely person who suggested that the refresh rate might be wrong. On checking through "Display settings" and the Advanced Display Settings and scrolling down I found Display adapter properties. And what d'you know? My monitor was set at 59Hz. Changing to 60Hz solved the problem in a flash. 

Try it - it might work for you, too!

BTW - who the heck decided that small, light blue text on a white background was the way to good communication?  My wife has eyesight issues, I doubt she'll even be able to see the colour, let alone read the text.

I had this problem shortly after going over to Windows 10. After some days, possibly because of an update download, it reverted to the correct ratio. All was well for several weeks until after a further update last night and now the problem has re-occurred. What are Microsoft playing at?  I believe yesterday's update included NIVIDIA so that could be the source of the problem

Half an hour after the above post I thought that I would try a restart. Guess what? All is now OK! What is the logic in that?

Here's what worked for me (I'm using another monitor because my screen is broken as well), and as for display adapter, I'm using the stock one: Intel(R) HD graphics family.

1. Navigate to the Control Panel through the start menu

2. Under "Appearance and Personalization", click "Adjust screen resolution"

3. Click on "Advanced settings" on the right hand side of the screen

4. Go to the graphics control panel tab (this will vary depending on what type of display adapter you're using)

5. Under "Scaling:" select "Customize Aspect Ratio", and set to desired preference.

Hope this helps!