How to change document resolution

Hi

I have created a whole bunch of psd files as using high quality graphics for Printables. But I accidentally didn't set it at 300 when I created the files. I have saved the files as both psd and jpeg. Now I want to print them but see they are actually at 72,! The clip art I used in the files were good quality to start with. It's me that set up the artwork composition files wrong. How do I fix it