First, the bad news: Mistakes happen—and when it comes to the three national credit bureaus, they happen a lot. A 2012 study by the Federal Trade Commission found that one in four people found errors on their credit reports that might affect their scores.

Now the good: "You have the right to challenge information on your credit reports with which you disagree,” says John Ulzheimer, a credit expert and Fair Credit Reporting Act consultant. “It’s going to be free and you can do it on your own.” It’s also highly likely you’ll be successful. Some 80 percent of people who filed disputes experienced some sort of modification to their report, according to the FTC.

Think something funny is going on with your credit report? Here’s what to do.

1. GET YOUR REPORTS.

You’re entitled to a free copy of your credit report from each of the three credit bureaus—Equifax, Experian, and TransUnion—each year by going to AnnualCreditReport.com. If you want to get it more frequently, you’ll have to go directly to each credit bureau and will have to pay a fee of about $15 for a mailed copy and about $23 to $25 for a real-time electronic version. You can also check your Equifax and TransUnion reports as often as you like without charge at CreditKarma.com.

2. READ CAREFULLY.

There are three types of credit report problems to keep an eye out for. Identity errors range from an incorrect address to a mix-up with another person who shares your name. Account detail mistakes include an incorrect credit limit, the wrong origination date on your mortgage, or a long-closed loan that’s still marked “open.” The last type, fraudulent activity, means that someone has used your personal data and Social Security number to open lines of credit in your name.

3. FREEZE.

If there’s an account on your report that you didn’t open, you need to immediately place a security freeze on your reports with each of the three bureaus. This can be done online and is generally free if you’re a victim of identity theft.

4. MAKE A FUSS.

You can file a dispute directly with the credit bureaus or with the source of the information (usually a bank or a debt collector), says Ulzheimer. The benefit of filing the complaint through the bureaus is that you can do it online; the downside is that they’re essentially middlemen and will need to reach out to the source. If you choose to go to the source yourself, it may expedite the process—but you’ll probably have to mail a letter rather than file your complaint electronically.

Either way, “the parties have an obligation under the Fair Credit Reporting Act to perform an investigation,” says Ulzheimer. "Sending the dispute to both parties doesn't ensure that it's going to be done faster or that the outcome will be different, but you certainly can burn the candle at both ends if you want."

5. SIT TIGHT.

Investigations will be completed within 30 to 45 days and you’ll be notified of the results, usually in writing. If the bureau or information source can’t verify the credit report item, it must be removed. If it’s verified, it stays.

6. DON'T GIVE UP.

If the dispute wasn’t resolved in your favor, don't despair. You can provide a short statement to the credit bureaus summarizing the problem; it will be included with your report and can explain the problem even if it’s not removed. If a valid error that hurts your credit is not removed, you can also escalate the issue by filing an online complaint with the Consumer Financial Protection Bureau. The bureau will forward your complaint to the provider of the offending information and will let you know if they receive a response.