Credit repair can involve fixing your bad credit in any way, shape or form, but when most people use the term ‘credit repair’, they’re referring to the process of disputing errors on credit reports. You can go through this dispute process for free with each of the credit bureaus on your own. This involves filing a formal dispute with the credit bureau(s) in question either online or via snail mail.
That formal dispute should provide a detailed explanation of the error and you’ll want to include any supporting documentation you have along with it. (You can learn more about disputing errors on your credit report here.)