To update logged sleep, meals, or diaper changes head to your Home page and tap 'View all history' to pull up your history in chart form. From there you can tap on any log icon and click the edit button that pops up on the lower right to edit the log or click the trash icon right next to it to delete it.
To edit logs that don't appear on the chart, click on the List view. If you're using an iOS device, simply swipe the entry you would like to edit to the left and you should see the option to edit or delete the log. If you're using Android, just click on the log and those options should pop up!