How do I delete the contacts from one group but keep them in my account?

In your dashboard, go to MY CONTACTS > CONTACTS & GROUPS.

Once there, you'll see your groups listed. Besides all groups, you'll notice a red delete button. Click that to delete the group. This will keep all of the contacts in your active contacts, but will remove them from that group.

If you want to keep the group and just get rid of the contacts to start fresh in that group, you can do so. Just delete the group and then create a new group with the same name.