The photo contest requires a resolution of not less than 300dpi. I shot 72dpi. Can I change it to 300dpi on my computer? I don't know.

but ...

As 1L said, DPI is the output resolution, which represents the number of pixels contained in a unit. The DPI of your original image is 72. When you change the software to 300, you actually make up for your missing pixel position by differential calculation. In other words, your actual picture quality has not been improved.

The 300dpi he asked for is a standard, and you can approve it when you meet it. But the reason why he asks for this standard is the requirement of image clarity, and your work actually does not meet the requirement of clarity, which will have a great disadvantage in grading your work.

By the way, 300dpi is the maximum clarity that a person with vision of 1.0 can feel when looking at an object 20 cm away. You are less than this difference, which means that your image has been blurred.