The following code illustrates how to rename a partition policy detail:

define variable policy as IPartitionPolicy no-undo. 
define variable detail as IPartitionPolicyDetail no-undo. 

assign
         policy = service:GetPartitionPolicy("OrderlineDatePolicy")
    detail = policy:Details:Find("OldData")  
    detail:Name = "OldDataDetail". 

service:UpdatePartitionPolicy(policy).