It may happen that, for mistake, you add (using the suggestion bar available in Windows 10 Mobile while you are writing) a not correctly written word: how can you delete it from the custom dictionary in order it will not be considered as a possible correct word in the future?
In Windows 10 you can manage custom dictionaries directly accessing some files available in your Windows User folder (e.g. C:\Users\myUsername\AppData\Roaming\Microsoft\Spelling\it-IT ) as explained for example in this site .
As a may be better alternative, you can act from Outlook as shown in the following screenshot and as explained searching in the Outlook help “Add or edit words in a spell check dictionary“: after reached the list of the available custom dictionaries, you have to choose each one to manage its specific word list.
However in Windows 10 Mobile those capabilities are not available in Office apps and you cannot neither edit proper files: the only options can be reached from Settings > Time & language > Keyboard > Tap on a Keyboard language but they are not worth for our purposes.
As usual, I asked for help in the Microsoft Community forum and, after some time, a good guy told me the solution that he found from an old post (13/10/2016 – What’s new in Build 14946) in the Windows Blog site => “Option to remove a word from user dictionary (Mobile): We learn from your typing on the phone and build up a local user dictionary that adapts the keyboard prediction, autocorrection and shape writing experiences to your language style. Sometimes you may type a misspelled word and send it as is, sometimes you may tap on a red squiggled word and then tap the “+” sign by accident. Both save the unwanted word into your user dictionary, and it may appear when you type something similar in the future. Now you have a way to manually remove bad entries: tap on the word you don’t like in the text box, and you will see a candidate that is the word with a “-“ sign before it. Tapping on that candidate will remove the word completely from your user dictionary and no longer suggest it“.
Unfortunately it does not works from Word or WhatsApp app but only with other apps like Cortana, Messaging (SMS): from so0me app (e.g. Word, WhatsApp) it is possible only to add (+ option) a word in the custom dictionary but there is not the possibility to delete one misspelled word (– option) you may added for mistake! Anyway, after deleting a wrong spelling word (inserted from whatever app) from the custom dictionary from one app that allows it, that word will not be available as right written for all the environment (all the apps).
Also in Cortana it is possible not only to add a word in the custom dictionary but also to possible delete it afterwards, as in Messaging app: therefore it is a feature available from the O.S. but, I do not know why, not used from all the apps …