What file contains user typed words (user dictionary)?
Direct Answer
Based on this link, I think the file you are looking for is:
    /private/var/mobile/Library/Keyboard/dynamic-text.dat

Look at the link above to see full instructions.

Original Post
Slow Way
To reject the suggested word, finish typing the word as you want it, then tap the x to dismiss the suggestion before typing anything else. Each time you reject a suggestion for the same word, the iPhone becomes more likely to accept your word.
Fast Way
Reset the keyboard dictionary: Choose Settings &gt; General &gt; Reset and tap Reset Keyboard Dictionary. Resetting the keyboard dictionary erases all words you’ve added, and you can start over with a clean slate.
Second Option (Apps)
Looking at this post, it seems that there are two apps that could allow you to do this.
AutoCorrector ($0.99) allows you to add custom words to your iPhone’s dictionary so they are recognized and suggested by the autocorrect. You can also add list of short-hand words you use in text messages. The app doesn’t recognize all words – certain swear words are off-limits, however it’s an easy way to train your iPhone to behave.
ManualCorrect Pro ($0.99) is available through the Cydia store and works on jailbroken iPhones. The app switches some of the autocorrect features, so that you have to tap on the autocorrect bubble for it to be activated. The app also contains the ability to turn on autocorrect features for correct punctuation, and correct capitalization. Like AutoCorrector, you can create your own list of custom shortcuts.
Links/References
https://discussions.apple.com/thread/2430621?start=0&amp;tstart=0
http://www.zeropaid.com/news/101147/6-ways-to-improve-your-iphones-autocorrect/