I always change my yahoo password every 3-4 months, this is for my safety. How to change password at your yahoo email ? follow this step below
login in to your email account and then find Option menu on top right.

and then after you klik option menu you will go to new page and find account information menu and the click it.

after that you will log in to account center and then find change password menu ..then click it.

and then yahoo ask your password to verify again before login to account information center.

and then after you click it you can change your password now, put your old password and then put your new password.