2 Answers

You may have the whole word highlighted. In that case, when you type another stroke, the highlighted part disappears.  Better to put the cursor just behind the letter you don't want and hit the "backspace", or just ahead of the miscued letter and hit the "delete".  Those choices will erase ONLY the letter and then you can type the letter you want and > to the end of the word and continue. 
If you highlight the entire word, you type the whole word and keep going.  

You may have the whole word highlighted. In that case, when you type another stroke, the highlighted part disappears.  Better to put the cursor just behind the letter you don't want and hit the "backspace", or just ahead of the miscued letter and hit the "delete".  Those choices will erase ONLY the letter and then you can type the letter you want and > to the end of the word and continue. 
If you highlight the entire word, you type the whole word and keep going.