Hello,
at the moment we are (mean privileged Avast staff) are able to do reset on cloud only. Please ask support for synchronization group reset or as noted many time here just send me an email to kvetko@avast.com from email address you are requesting reset for with message state you are asking reset and I will perform it for you. Note that before you ask the reset you should disable synchronization you have that enabled on (no matter state - assuming that you request reset only in case you don't have any trusted device to confirm your new one.
Reset will result to loss of the data you have stored in the cloud and the operation cannot be be reverted!