To change the image resolution in Adobe Photoshop follow the instructions:
Step 1. Select the command Image - Image Size from the Adobe Photoshop menu.
Step 2. Select the option Resample Image, if you want to keep the image dimensions (Document Size).
To keep the total amount of pixels (Pixel Dimensions), you should un-check this option. In this case the width and the height will change so the total amount of pixels will stay the same.
Step 3. Enter 300 (or less, just not below 120) into the field Resolution (which defines how many pixels per inch your image will have).
Step 4. Press the button OK.
Caution: You cannot simply convert a low-resolution photo to a higher resolution by increasing the DPI in your imaging program. The printed result will be a blurry image.