On the Speed Dial screen a new entry (highlighted) will automatically be placed into the first free speed dial number and you can:
Accept the number: press to accept the speed dial number.
Change the number: use /
to move the highlighted entry to a new speed dial number and press
to accept it. This over-writes anything already in the speed dial number.
To invoke a 1-digit speed dial, press and hold for a few seconds the key for that digit while on the idle screen. To invoke a 2-digit speed dial, press (and release) the first key, then hold the second key.