The following code shows how to edit an existing policy:

define variable policy as IPartitionPolicy no-undo.
assign
   policy = service:GetPartitionPolicy("OrderlineDatePolicy")
   policy:Description = "Orderlinedate policy").
service:UpdatePartitionPolicy(policy).

You can alternatively access the policy from the table:

define variable tbl as ITable no-undo.
assign
    table = service:GetTable("Orderline")
    table:PolicyDetail:Description = "Orderlinedate policy").
service:UpdateTable(table).