It is pretty easy to file a credit dispute. The website of each of the credit bureaus has a simple point and click dispute feature that lets you select from a short menu of explanations for your dispute ranging from "not my account," to "never late." Explanations that fall into the category of "excuses" (such as couldn't afford to pay) will do nothing to remove the negative information, so don't waste your time going there.
Filing a credit dispute doesn't guarantee that the negative item will be removed, as the credit bureau can take 45 days to investigate the dispute before they remove any item they cannot verify. The credit bureau relies on information from the creditor to verify their records. The older the information being disputed is, the more likely it will be that the creditor will not be able to produce the records to verify the matter disputed.