Read the letters you want to use and make sure they will help the medical school see you as a good candidate. Mentions of strength in science, anatomy or work ethic are good signs.
Call the person who wrote your reference letter and ask permission to reuse it for a different medical school application. The medical school may contact the person for further information, so it's best check with the letter writer beforehand. Ask if he has a copy saved that he can update with a current date. Have the person email the file to you so that you can print it and use it.
Contact the medical school admissions office. Explain that you have a reference letter that supports your application but that it was originally written for a different application. Ask if it breaks any rules to reuse it.
Include the letter of reference in your medical school application if you have permission to reuse it.