Changing the email address linked to my Steam account
Solution 1:
You can change your contact email address via the steam client. Menubar -> Steam -> Settings -> Change Contact Email Address.
For security reasons you will need to log in to steam and have access to the old email address.
Solution 2:
You can only change your Steam email account while logged in.
However, it is possible to access Yahoo mail from any computer using the web interface at https://mail.yahoo.com/. I hope you remember your Yahoo mail password!