Windows 10, and most prior versions include Paint as an installed application. It will do resizing for you. Find the image you wish to resize using Windows Explorer and Right-Click on it. Select Edit from the menu that pops up. This will launch Microsoft Paint.
PLEASE NOTE THAT THIS IS AN OLD GRAPHIC. PPI NO LONGER MATTERS AND THE LONG EDGE SHOULD BE 1440 PIXELS
In the toolbar below the main menu bar there are sections in what Microsoft calls the Ribbon. You may need to make the window wider to see the words in the Ribbon. Select Resize in the Image section. This will give you the Resize dialog box. Make sure that the Pixels section is checked and then change the largest number in either the Horizontal or Vertical boxes to 1280. The other will follow automatically so long as you have not unchecked Maintain Aspect Ratio. Click OK and Save the updated file.
There is no simple and free way to change the ppi resolution in Windows. If you are using Photoshop, Lightroom or Elements they all do this as do other Windows paid for applications such as ACD See. While I would prefer you to explicitly set the PPI to 125, we can cope with a higher or lower number for the web.