Mark top left corner with C-<Space>, place cursor on bottom right corner and use:
C-x r t – replace selection
C-x r k – kill selection
C-x r y – yank (paste)
In command mode, start selection with C-v, expand it using the arrow keys and use for example y to copy or d to delete. For line selection, use shift-v instead.
Press Ctrl, left-click and make a selection with the mouse.