For the past year I have been erasing entire words when I wanted to correct an error in the middle of the word. I thought my finger was too big to get the cursor to line up on the letter – the cursor would only go to the start or end of a word.
Apple has thought of this – what you need to do is hold your finger on the word that you want to correct. An image that looks like a magnifying glass will show up – keep holding your finger on the word, and you can now slide the cursor to the exact letter that you want to correct. When the cursor is in the correct position, lift your finger off the screen and start your corrections