When it comes to image resolution, DPI by itself has very little meaning. This is usually a meta tag embedded in an image file that stands for dots-per-inch and is only relevant when you know the print or display size of the image. That means even a high-resolution photo can be rejected for publication, for example, if they are only looking at the DPI value in the file details. It's very easy to change DPI in apps like GIMP, Paint.NET and Photoshop. However, if you really don't want to install another app, you can change the DPI in Microsoft Paint, provided you have another image that has the DPI you need. Keep in mind that changing the DPI by itself does not enhance or reduce image quality.
Using MS Paint
Open the image you want to edit in Paint. Select "Properties" from the File menu and make a note of its height and width. You may notice that Paint also lists a Resolution DPI. This number is not reliable and should be ignored. Depending on the file format and which computer you're using, Paint may show you "300dpi," "120dpi," or even "96dpi" for the same image.
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Locate an image in File Explorer that has the DPI you need. Right-click the file, select "Properties" and click the "Details" tab. If the image is a JPG or TIF, Windows 8.1 shows you the DPI along with its dimensions. Right-click this file, select "Open With" and select "Paint." This second image opens in a new window.
Click the "File" menu in the second Paint window, select "Properties" and then change the height and width to the same dimensions as the first image. Click "OK."
Go back to the first Paint window with the low-resolution image. Press "Ctrl-A" to select the image, then "Ctrl-C" to copy it. Click the second Paint window and press "Ctrl-V" to paste the first image onto the high-resolution image.
Select "Save As" from the File menu and save this as a new file with a different name so you don't write over the original image file. This new copy keeps the DPI level used by the high-resolution image, which you can confirm by looking at its file properties.
Using Other Apps
Install a photo editing app like Paint.NET, GIMP or Photoshop. These each have an option to change an image's DPI within the file itself. Paint.net and GIMP are free to download. Photoshop is available as a free 30-day trial.
Open your image with the new app. Click the "Image" menu and choose "Scale Image" in GIMP, "Resize" in Paint.NET or "Image Size" in Photoshop. In GIMP, you need to change both the "X Resolution" and "Y Resolution" to the DPI number you need. In Photoshop and Paint.NET, just change the single "Resolution" field.
Save the image as a new file so you don't write over the original version. In Paint.NET and Photoshop, use the "Save As" option from the File menu. If you're using GIMP, select "Export" instead.