How to remove a word from the custom dictionary
To do this, follow the next steps:
1. On the File tab, click Options:

2. In the Word Options dialog box, on the Proofing tab, under When correcting spelling in Microsoft Office programs, click the Custom Dictionaries... button:

3. In the Custom Dictionaries dialog box, select the dictionary by default (with high probability, the added word will be in it), and click the Edit Word List... button:

4. In the dialog box, named as the dictionary you have chosen in the previous step, select the word you want to remove and click the Delete button:

5. Click OK.
Notes:
- When you work with some document on different PC, laptops, etc. or if this document should be checked by somebody else, you can create your own dictionary just for this task (paper, report, project, etc.), copy and send it to other place or to other people for future use.
- Added word can be saved in different dictionaries. With high probability, the added word will be saved in the dictionary that is marked as dictionary by default. In some cases, you can need check other existing dictionaries to remove that word.
See also this tip in French: Comment supprimer un mot du dictionnaire personnel.