The usual hot keys work in the editor:
Shift + arrow for select
Ctrl + C for copy
Ctrl + X for cut
Ctrl + V for insert
Ctrl + Z for undo
Ctrl + Y for repeat (available only in case of no other edits occur since last undo)
Of course you can also select with the mouse and use your browser's editing features.
Linux and other UNIX OSes / X11: A left doubleclick selects and copies a word, and a click on the middle mouse key is insert.