How can I create backup of Yahoo mail account?

You could use a email client app (e.g., Thunderbird, Outlook, mobile email client apps, etc.) with you account configured as IMAP to allow the server (mirrored by the webmail shown in its mailboxes) to sync with your client and that would mean it would download all the mailboxes to your client on your computer.  IMO that's "good enough" but if you want to store the emails on your computer in some other form outside the client then you will need to google search how to extract the email from your client and how you do that depends on which client you use.

@Brianna131

What's the advantage or point of using a extra third party tool to download the webmail into a pst file that you then have to stick into outlook when you can download the webmail directly into outlook (or any other email client the user might prefer) itself (no "conversion" necessary)?  It's not going to be any faster since the download time will swamp out everything else anyhow.