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While there is often a DPI stored in an image’s EXIF data, images themselves don’t have a DPI (you can learn more here).
DPI refers to how many dots there are per inch when printed, and the smaller you print an image, the higher the DPI will be. So, for example, a 3000×3000 pixel image will have a DPI of 300 when printed at 10×10 inches. When printed at 5×5 inches, the same image will have a DPI of 600.
So, basically the only way to increase an image’s DPI is to increase its size. Normally however doing so will decrease the quality of the image, and so there isn’t any benefit of just resizing the image larger.
Let us know if you have any further questions.