You can set the PPI to any value you want. That value is meaningless until you print.
In Photoshop, open the image. Click (from the pull-down menu along the top) "Image" -> "Image Size..."
Important! UNCHECK the box that says "Resample" (if it is checked.)
Type in the box that says "Resolution" the new value that you'd like (e.g. 300) and make sure you've selected "pixels/inch" (usually that's the default anyway.)
Click "Ok"
That's it... you're done.
What did this do to your image? Absolutely nothing! Not a single pixel was harmed in this process (because you disabled "resample"). All it really did was change a meta-data label that used to declare that you had a 72 dpi image to declaring that you now have a 300 dpi image. But did you really change anything.... nope!
That's because "pixels per inch" is a printer term. If I want to print a 4x6" print (and I physically want it to be 4x6" and not any other size) AND my printer is set to print at 300 dpi, then I need a pixel with dimensions 4" x 300 pixels per inch for a total of 1200 pixels in that dimension and 6" x 300 pixels per inch for a total of 1800 pixels in the wide dimension and I'll get the exact output size I want.
But sometimes a publisher says 'I want your picture to fit in a 3" x 4" spot on the page and I'm printing at 300dpi.' What that means is they want YOU to work out the math to realize that your image needs to be 3" @ 300 ppi (900 pixels) by 4" @ 300 ppi (1200 pixels) and in that case you would tell Photoshop to "resample" the image to a new resolution of 900 x 1200 and you'd set it to 300 ppi and when it goes to the printer it will exactly fit in the space they want. The resampling process will cause Photoshop to eliminate (or add) pixels (make sure you save your original image because Photoshop is a "destructive" editor.)
