Edit Autocorrect on iPhone by tapping the suggestion that appears in the predictive text field. This will teach iOS autocorrect new words and spellings.
Accept the iPhone spelling suggestion by typing a space or a punctuation mark or tapping the word.
If you accidentally accept an autocorrect suggestion, you can edit Autocorrect on iPhone by tapping delete and retyping the last letter of your word and the suggestion will go away.
Sometimes Autocorrect offers additional suggestions. If you accept too many erroneous corrections, Autocorrect may start correcting words you don't want changed. If this happens, you can edit iPhone dictionary by going to Settings > General > Reset > Reset Keyboard Dictionary.