Open the settings on your phone and then go to the Samsung account settings on your phone from there you can change your password and also check what devices you are logged into currently, after you change it will automatically change the Samsung members app password
You should see an option to change your password, You need to enter the Current password in order to change it. If this doesn't work open the settings on your phone and then go to accounts and backup then manage accounts and remove your Samsung account and try to re add it
Have you tried to remove the Samsung account from your phone and then re-add the account, if you don't have the correct password for your account it will not let you change any settings, if you don't have it click on the forgot password link to receive an email or text message to reset your password then try again
