Eat as normal up to the fourth day before the marathon.
On the third day before - start carbo-loading, with increased portions of carbs, such as pasta, bread, etc. Modest amounts of protein are OK; reduce the amount of fat you are consuming. Avoid alcohol.
Continue this for the next two days - though I've heard that the first day is most important.
On the last day before the race keep yourself well-topped up with food during the day, but with your last full meal about 12 - 15 hours before the race start time. Stay well-hydrated.
On the morning of the race have a fairly light breakfast at least 3 hours before the start - something like bread with jam or honey or porridge are good choices. Avoid fruit, fruit juice, or anything that might uupset your stomach. An energy drink might not be a bad idea as well.