Let’s say you want to pixelate a certain part of an image, you can do that in Photoshop.
1.) First, select the layer of the image that you want to edit.
2.) Select the “Rectangular Marquee Tool”. (You can also use the elliptical one)
3.) With that tool, select from the image which part of it you want to edit by click-dragging on the area.
4.) While the area is still selected, go to Filter > Pixelate > Mosaic
5.) A Mosaic panel will pop up and you can adjust the quality of pixelation by dragging the “Cell Size”.