Background changes mainly use matting. There are many matting methods in PS, such as magic wand matting, polygon lasso matting, pen matting, channel matting, filter matting and so on.

The easiest way is to use the magic wand to select the background color, adjust a color you need in the foreground, and then press ALT and DELETE keys at the same time, and the color will be replaced by the color you need. Very concise, I hope you can understand.