Unless you opted to hide spelling errors in notes, you can see where spelling errors are because they are underlined in red. To fix spelling errors, you can either right-click misspelled words and correct them one at a time or run the spelling checker to examine the spelling on a single page.
The spelling checker occasionally flags a word that is correctly spelled. You can ignore these words or add them to the spelling dictionary. Adding a word to the spelling dictionary instructs OneNote that the word is legitimate and should no longer be considered a misspelling.
Right-click a misspelled word (a word underlined in red).
In the shortcut menu that appears, choose the correct ...