Request credit reports from annualcreditreport.com, and check the accounts for errors. Note that there are some errors that you may remove and not actually improve your credit score, for instance, a name misspelling. Still, go after all errors despite how serious they look. Then visit the respective online disputing section, and file your dispute.
The major credit bureaus allow you to submit your request by mailing in a physical letter. You can use a 609 template to easily come up with your dispute, but generally, your letter has to mention the credit error and why you need it resolved. The credit bureaus also allow phone disputes.