PHOTOSHOP - Changing image size

To change the image size or resolution in Photoshop:

In the menu bar > Image > Image Size

Under Document Size you can change the width and height of the image.

To decrease the resolution to 72 ppi (for screen) simply change the resolution to 72ppi.

 

To increase the resolution to 300 for printing you first need to uncheck the Resample Image box. Then change the resolution to 300ppi. You will notice that this decreases the physical size of the image. This needs to happen as you cannot simply add pixels (information) that don't already exist to images. 

This is ok, as it means that the image will now print and be the best quality print (albeit a little small).