The auto-complete features can be nice, but somewhere along the line my iPhone has learned some questionable language. How can I get the iPhone to "forget" certain words from its auto-complete settings / dictionary?
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:
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!
You can either erase all learned words by going to Settings > General > Reset > Reset Keyboard Dictionary, or you can have your iPhone automatically censor out bad language that you type. To do the latter, go to
Settings > General > Keyboard > Edit. You can then set a "censor" for bad language. What you could do is create a new shortcut for when you type the Phrase "cheese", it'll automatically Shortcut to "ch***e."
Short of a reset (as the other answers mentioned) or jail breaking, you can't :-(
There are a lot of tips to tuning it in this GigaOm article (including the name of the file to edit if you do jailbreak).
You can tune it by typing a word and insisting it is correct, you can add a word manually Through a somewhat hidden feature in international keyboard settings, but annoyingly the only way to make it forget is to forget everything.
I had an issue where every time I typed the name "Larissa" it would capitalize it to "LARISSA". No matter how many times I hit the "X" next to the suggestion, the thing just would not go away.
I discovered that I had a contact in my address book with the surname "LARISSA" (capitalized), and it was using this contact to make the correction automatically. As soon as I adjusted the contact, it no longer forced this capitalization.
It's possibly a rare case, but hope it helps someone else out there.
(Using iOS 9.3)