Double-check that what you have encountered in the record is actually an error. Discuss your findings with the patient, charge nurse, or doctor. If you are not the person who originally recorded the finding then attempt to discuss the discrepancy with them directly.
Write a single line horizontally through the incorrect information. Do not scribble over the writing to obscure what was originally written. Do not attempt to erase. Never use white-out. It is important that the original record remain legible for medicolegal reasons.
Write your initials and the current date next to the incorrect information. This serves as a record of the source and time of the correction. This may be very important if mistakes were made during patient care based on the original mistake.
Write the correct information legibly as close to your initials as possible. Resist the temptation to write 'error' next to the correction, or in any way attempt to explain the mistake. Stick to replacing the inaccurate information with accurate data.
Inform any coworkers involved in the care of the patient (if you did not already speak with them previously.) It is unlikely that another healthcare worker will read through the entire record and notice the correction you have made. As the person who identified the error, it is your responsibility to make sure everyone is on the same page.