You can open the full spell checker from the Symbols tab, or by pressing F7.
The current misspelling will be highlighted in blue and show in context in the top box. The second box contains suggestions for the correct spelling along with symbols.
If the correct spelling appears in the suggestions, select the Correct button to update the document. You can also select Correct All to changes all instances of that spelling error in the document.
If the suggestions do not contain the word you are looking for, you can search by typing into the Custom spelling box and clicking Try this. This will populate new suggestions based on the searched phrase.
If the marked error isn't a spelling mistake, for example a surname the dictionary doesn't know about, you have additional options.