Changing PPI

[Gh0strider]

When you upsample an image in Photoshop from 72 ppi to 300 ppi, the image gets much larger? Where do the extra pixels come from without causing interpolation?

[r-bin]

Poorly understood what you are asking.

Can this article help you?

Difference Between Image Resizing and Resampling in Photoshop

[JJMack]

Resampling an image creates a new image that has a different Canvas size. Print resolution PPI is just a printer getting that tell the Printer what size to print  an image's pixels.  You do not have to reasmple an image to print it different sizes you can just change the print ppi resolution setting without resampling the image. Print resolution is meaningless on Displays. Displays have a fixed resolution. All that matters on displays is the Image canvas size. If the canvas size is larger the image may not fit on many displaye.  Displayes display a fixed number of pixels Photoshop's zoominf scales you image to a different canvas size. You are only viewing you actual images pixels when you zoom to 100% actual image pixels will be displayed.   

If you change and image's print resolution from 72ppi to 300ppi without resampling the images to a larger canvas size the 300ppi print will be much smaller then the 72ppi print. 

Photoshop shows that here at 72 PPI the print will be a 6" x 9" portrait at 300 PPI the portrit print will be 1.44" x 2.16". The image canvas size is 432 px x 648 px a good web size image. It will fit on all web devices displays except very small displays like an Apple watch..

[D Fosse]

They don't. Interpolation is exactly what happens (and going from 72 to 300 will ruin your image).

Uncheck resample. That leaves the file unchanged, only the pixel density on paper changes, and so the physical print size changes.

Resampling is safe when going down - as long as you keep the original! - but not up.

[c.pfaffenbichler]

As pointed out the assumption in your question (»without causing interpolation«) directly contradicts your previous statement (»upsample an image«). 

Maybe you could explain what you are actually doing or trying to achieve (possibly with the help of screenshots)?