Under account information in the main menu, you should see a button labeled ‘Change Password’. If you have a verification email already linked, you can press this button to receive a form to change your password. Make sure that the email you receive is from Pixel Worlds, then go about changing your password.
If you are stuck out of your account and do not remember the password/email associated with your account, send an email to the one linked above. Explain your account name, when you were locked out of your account, and the issue of you forgetting your password. To help your case, you can also include images of previous in-game purchases that you may have made through your account in order to verify that you are the true owner.