This articles demonstrates how to modify spellcheck settings.
By default, you should have spell checking turned on in the EHR. If you'd like to disable it (or you've accidentally disabled it and would like to re-enable it) just right-click in the white-space of any encounter note and you should see a menu option for "Spell Check".
Hover over this and make sure that "Use basic spell check" has a check next to it (or uncheck it if you'd like to disable spell-checking):
Expanding the Spell Checking Dictionary
If the browser is regularly flagging words that are spelled correctly, it's every easy to add these to your dictionary. Just right-click on the underlined word and select "Add to dictionary":