I found the answer in the very useful but well hidden Advanced User Guide at
https://www.bt.com/content/dam/bt/help/digital-voice/advanced-phone/BT-Advanced-Digital-Home-Phone-U....
Editing a speed dial entry
1 Select the Contacts list, then the right navigation
button.
2 Scroll to the speed dial entry you
want to edit and press Options.
3 Scroll down to Edit speed dial and
press Select.
4 Follow the on-screen steps to
change the speed dial entry.
Press Select to save your change.