1. First, open the PS in the computer and import the picture whose background size you want to adjust.

2. Select "Lasso Tool" in the left toolbar, and select the area that you don't want to deform, such as the figure below, so that the selected part will not be stretched.

3. Right-click to find the storage selection, and then customize the name to store this section.

4. Press Ctrl+D to cancel the selection, and select Edit-Content Recognition Scale in the menu bar.

5. Finally, stretch the background of the picture, and the previously selected area will not be deformed, thus completing the operation.