Log into DMB at diamond-mind.com and click on the Forgot my password link.
On the next screen that appears, enter your e-mail address (this address MUST match your DMB account e-mail address). Then click on the Submit button.
You will then receive an e-mail from DMB/Shopify that contains a link to reset your password. Click on this link
Once you click on the link you receive, on the next screen enter a new password, confirm your password, and then click on Reset Password.