Yes, you can. Just log into your account and click on "Membership Settings", "Change or Cancel My Membership". On the right side of your screen, you will see a red button that says "Cancel Membership". Click on it, and that's all!
You'll have access to your materials until the end of the billing term, and you won't get billed unless you reactivate your membership.
You can access your Membership Settings in your CharityHowTo library at this link: http://www.charityhowto.com/library
If you need to you can set a new password at this link: https://charityhowto.com/user/forgot-password