I'm trying to massively convert Leads into Accounts and Contact. Could create Contacts and Accounts with no problem. When I tryed to update the Leads I received and error saying that certain fields could not be updated. The error says:

"Unable to create/update fields: ConvertedDate, ConvertedContactId, ConvertedAccountId. Please check the security settings of this field and verify that it is read/write for your profile."

How can I do that? Is there any other way of doing this? I also had trouble trying to set the "Converted" field value to "true" because it said it was not a valid value. Should I use number 1?

Any advice will be very appreciated.