If you're logged in to your Revv account, go to your donor profile at app.revv.co. You can change your password by choosing the Password option from the menu. Enter your current password and then enter your new desired password. Make sure your new password is at least 8 characters. Finally, re-enter your new password, making sure that it exactly matches the new password you've just entered.
If you've forgotten your password, you can reset it here. You can also reset your password on any Revv donation page by clicking the Login button, and then clicking Forgot your password?