How to modify unsupervised dedupe rule
On our site we simply created a new rule, and chose the 'Unsupervised' option when creating it.



That creates the rule, and marks it as the default 'Unsupervised' one. The old default is downgraded to a 'General' rule. If it was a built-in rule then it stays 'reserved' so you can't edit it.

I don't think there's any need to delete them, or fiddle in the database =]