How do I delete foods

I dont think you can. They just drop off the end of your list as you dont use them anymore and you put new foods on that you do use.

You can't. And it's frustrating, so MFP developers, if you are reading, please change this!

I still have food that I ate only a couple of times, years ago.

I assume you are asking about the Recent list. ​The Recent list is just a database query that is generated each time the Food Diary is searched and displays up to the last 100 items you, yourself, added to your Food Diary, either by individual Meal, or by All Meals, depending on your display option. Those food items will eventually drop off of the Recent list display after you log more items. You can not delete them from the Recent list.

The Frequent list displays up to 25 items from your Food Diary based on an algorithm that the user can not change, but a user can in effect hide particular food items from displaying in the Frequent list.

If you want, you can change your default display to the Frequent list, from which you can hide previous food items that you added to your Food Diary and do not want to see displayed on a list of foods. On the Web version, navigate to FOOD > Settings and change Default Add Food View to Frequent. On the mobile apps, navigate to Settings > Diary Settings and set Default Search Tabs to Frequent.

The only way to have the Recent list not display those items that you added to your Food Diary is for you to go back to your Food Diary and delete all instances of those unwanted items from every day that you logged them. You can go back to individual days in your Food Diary and delete the food items one by one on the Web version or by multi-delete on the mobile apps. However, if you delete those unwanted entries from your Food Diary, be aware that your macros will not be correct for those previous days and any Reports, if you use that, will not be correct.

​Please see these articles, and search for other helpful articles, on the MFP Help pages...
myfitnesspal.desk.com/customer/portal/articles/374439-how-do-i-delete-an-entry-from-my-food-diary-
myfitnesspal.desk.com/customer/portal/articles/152746-can-i-remove-a-food-from-my-%22most-used%22-or-%22frequent%22-list-what-if-i-change-my-mind-