If you are able to log into your Square Enix Members account, click on your username in the top right-hand corner of any Store page and select Settings from the drop-down menu. Then, click on the Change Password option towards the bottom of the page.
If you have forgotten your password, please click on ‘Can’t access your account?’ on the login screen and enter the email address registered on your Square Enix Members account. You should then receive an e-mail with instructions on how to reset your password.
If you log in using an alternative method (such as your FINAL FANTASY XI or XIV account, Google, Steam, Facebook, etc.) you will need to reset your password on that account system first. Your Store account password will then automatically update when you next log in using that account.