On the Speed Dial screen a new entry (highlighted) will automatically be slotted into the first available slot and you can:
Accept the number: press Select to accept the slot number.
Change the number: use /
to move the highlighted entry to a new slot and press
Select to accept it. This over-writes anything already in the slot.
You can also:
Add an Action: press Select and choose an action to add to a slot. OR press
Options and choose Set... then choose and action.
To launch anything that has been speed dial-enabled from the Home Screen, hold down the number button. For 2-digit list entries, hold down the second number button.