You can change the Access Code for your account by logging in and clicking the Settings tab, then choosing Access Code in the left-side menu. Click on the 'Get New Code' button and then in the pop-up window, confirm the change by again clicking on Get New Code.
Changing the access code here will impact ALL of your upcoming calls. So if you have any meetings scheduled already, be sure to notify all participants of the new code.
*Note that it is not possible to have your previous access code reinstated once it has been changed