How do you change the resolution of an image in Photoshop?

Go to "Image", then from the drop-down menu, choose "Image Size". In the new box that opens, you can change the width and height of the image, by pixels, or by a percentage, as long as you have "Resample Image" checked at the bottom of the box.

If you want the image to maintain it's correct proportions when you scale it, then check the "Constrain Proportions" box. This will ensure that you don't change the size of the width, more than the height, or vice versa, which can lead to an image which appears stretched or deformed.

Under the "Resample Image" box, is a drop-down menu, in which you can select different algorithms for increasing or decreasing the image resolution. Different algorithms are more suited for different tasks, with hints provided in parenthesis as to what each algorithm is more suited for.

Example: "Bicubic Smoother ( best for enlargement )"

Note that increasing the resolution of an image will not increase the detail, so making a very small image larger will make the image appear stretched, blotchy, or pixelated.