I just found out that my account is being used by another person?
Change the password.
📌 Note that a password applies to all of Yahoo. There is no separate password for individual sites.
📌 If you know your password:
You can change password by going to https://edit.yahoo.com/config/change_pw or hover over the far right gear icon ⚙ and select "Account Info". Then click on "Account Security" and "Change your password".
📌 If you do not know your password:
If you forgot your password or Yahoo ID go here:
📌 Note that there is no way to retrieve an old password. It must be reset.
Secure a hacked account - https://uk.help.yahoo.com/kb/account/secure-account-sln3516.html
Check for a mail forwarding address and account activity.