Changing login method is only applicable if your account was created through Social Accounts (e.g Facebook or Google) and process is quite straightforward.
1. Just login to your account first using the social account associated with your account. You can login at https://simplywall.st/welcome
2. Go to your user Profile which you can access on the upper right corner menu. Then;
3. Look for and click on the Convert to email login button.
4. Set password at the bottom and save.