Apple - How can I get the iPhone to "forget" certain words?

It sounds like it used to be possible to manually edit the user dictionary, but unfortunately it isn't anymore.

Just a guess, but perhaps you can have it forget them the same way it learned them — when it suggests the word, try tapping on the small [x] next to the suggestion. If you do this a few times it may "forget" the word.

You can reset the dictionary entirely by going to Settings > General > Reset:

I opened notes, typed the word I wanted and when the incorrect autocorrect showed I pressed the cross. I pressed space bar and repeated the process 8 times. On the 9th time the incorrect suggestion stopped showing. Now it doesn't suggest the incorrect word anywhere!

Go to Settings--General--Keyboard--Add New Shortcut

Put the CORRECT word in the phrase spot and the misspelled word in Shortcut spot. Every time you type the word the phone wants to autocorrect with the misspelled word, it will automatically default to the CORRECT spelling! It doesn't remove the word from the dictionary, but it beats the phone at its own game!