Sharon
Yahoo Mail doesn't have spellcheck. You may have a spellchecker built right in your browser you can use. Check your browser settings to activate it: https://help.yahoo.com/kb/SLN3593.html
?
Spell check is now in web browsers
Your web browser will highlight errors and many will offer suggestions for correct spelling.
https://help.yahoo.com/kb/mail-for-desktop/spell-check-yahoo-mail-sln3593.html