Changing login method is only applicable if your account was created through Social Accounts (e.g Facebook or Google)
Process is quite straightforward. Just go to https://simplywall.st/resetpwd then enter your email. You may be prompted that the account was created through Google social login, just click convert to local account if button is available otherwise just click continue button.
An email will be sent to you containing a link to set your own password. Kindly check your junk/spam folder if it is not in the main inbox.