Unfortunately it seems that there isn't any "standard" way to reset the dictionary in Windows 10 Mobile. Hopefully this will be added back in soon. I don't want to try this for fear it will actually work, but you could try adding a different keyboard, removing your default one, then reinstalling it.
UPDATE: A post recently published by Microsoft states that ...
Go to Settings > Time & Language > Keyboard and tap on your keyboard in the list of keyboards. There you will find all autocorrect options. I believe "Correct misspelled words" is the setting you want to turn off.
Here's a related question.
It's not the same on Windows 10 Mobile but here's what I found:
Selecting on the word after it was auto-corrected suggests my spelling.After choosing my spelling the force-correct for that word appears to be disabled for the current email. I hope it's the same for other document types.
Tapping right after the word, before it's ...
Not possible in this release, sorry.
If you want to send feedback to Microsoft about this, I've listed a number of feedback routes for wp on this page - http://dfwiki.devfish.net/technology.Microsoft-feedback-routes.ashx .
Windows 10 Mobile sometimes deviates from the settings of WP8.1 and it is very common for Bluetooth as well. What you can do is:
Update your phone to the latest build of OS.
Make sure that keyboard stuff for your selected language is downloaded. If not go to language settings and add that keyboard.
Long tap the lower right toggle key of keyboard then go to ...
You can change the word's case by selecting it and then press the SHIFT key on your keyboard. So let your device write "LinkedIn", select the word and change the case with SHIFT until you get "linkedin". I often use it on my Lumia 1020 because it always suggests "Smartphone" instead of "smartphone".
I have the same problem, it's so annoying. I don't want to turn off autocorrect, I just want it to stop autocorrecting correct words to stupid words, eg long to Lon, if to I.
Just typing normally, not WordFlow.
Language is English (New Zealand)
Keyboard is English (UK)
I've just reset suggestions under keyboard\advanced, and retyped the same message (in ...
One workaround is to use accented letters where there should be any. For example, replacing the u in F*ck with ū results in a word which can be added to the dictionary and will happily be suggested by the keyboard.