Using the Selection tool, select the part of the picture you want to get rid of, then hit Delete.
Go to your other picture and, using the Selection tool again, select the part of the picture you want to 'import', this time go to Edit > Copy. Then go to the first picture, and go to, Edit > Paste > Paste as a new layer (Ctrl + L, does the same). Then move the new piece around with the Move tool until you get it in place. If need be, bring up the Layer Palette and move the new piece's layer under the original picture's layer.