How do I change one secret key on my gateway without retyping all the others?

Secret fields are never shown again after you save them. They appear as dots with the hint "unchanged — leave blank to keep".

1. Go to Store Settings → Payment Gateway and click Configure on the gateway.
2. Type the new value only into the field that changed.
3. Leave every other secret field blank and click Submit.

Blank secret fields keep their stored value, so nothing else is lost. The same applies to Webhook Secret (Optional).

If the gateway row shows Add keys instead of an Enable tick box, the keys for the selected mode were never saved. Open Configure, enter them and click Submit.