It should not be difficult to correct an error on your credit report. You don't need to hire anyone to help you!
Just send a letter to the credit bureau that is reporting it, and dispute the error. You do not have to dispute the item with the creditor themselves, although it doesn't hurt to send a copy of your dispute to the creditor. It's recommended that you send your letter via certified mail, return receipt requested. That way, you have proof that it was received, and when it was received.
Include copies of any documents that support your position, like receipts, canceled checks, or other proof of payment. For an example of how to do it with instructions, visit the FTC website.
Credit bureaus are required by law to investigate your dispute and respond to you in writing within 30 days. If they make a change to your report, they must also send you a free copy of your corrected report.