Here is the easiest way to remove a word that you added to your dictionary:

 

In my example, I have added the word "incorrectlyadded" to my dictionary.

1. Select Open and then Options:

    

2. Click on Spelling:

    

3. Click on Custom Dictionaries:

    

4. In the Bottom Left section named, User Dictionarie, click on "userdic.tlx" then click on the button named, View Words".

    

5. Find the word you need to delete, select it, then press the "Remove" button (which looks like a red octagon with a white minus sign inside):

    

NOTE! If you have added many words to your list, it might be difficult to find the one you accidentally added. The words are roughly in alphabetical order, but
they are in batches. So, you might get a batch of words in alphabetical order, if you do not see your word within the area it would normally be alphabetically, look further down the list to see if there is another section that it could be in. You can also start typing the word while in this window and it should jump down to that entry.

6. After removing the word, click close or OK on the windows that are open and the word should appear as misspelled again when you type.