This might be a cover letter, providing corrected information or addenda to previously supplied information. Apologize for any inconvenience it may have caused.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I'd like to apologize for giving you <incorrect/incomplete> information. <Description of the problem.>. <Solution.>.
I'm sorry for any inconvenience this may have caused. I appreciate your understanding. Please call me if you have any questions. I can be reached <contact information>.
encl: <List of enclosed items goes here>