Photoshopについて

Hi there,

The workflow may change/depend on the end result you wish to achieve. If you simply wish to replace the background of a selection with a different image or a solid color, once you have made the selection using the Lasso tool or any other selection tool, select the layer and click on the Mask button at the bottom of the Layers panel. This would keep the selected portion of the image and hide the rest. Add the layer with the background and ensure that the layer with the selection is at the top. 

You can alos check: https://helpx.adobe.com/photoshop/how-to/remove-replace-background.html

Regards,

Nikunj