Locate the ruleset (Where is my ruleset?), expand the ruleset node, the Rules folder and in the context menu of a rule node, choose Rename.

or

Locate the flow ruleset (Where is my Flow Ruleset?), expand the ruleset node, the Rules folder and in the context menu of a rule node, choose Rename.

In the dialog box that appears, the existing name of the rule appears in the field. Enter the new name for the rule in the field. Choose OK.