If you need to change a contact from group, go to the client's profile and at the buttom you you'll find the "Change group" option. You can edit this as many times as you need. 

If you don't  know the group the contact belongs to you can take a look at this image with the explanation by each specific group:

As of right now there is no way to batch changed contacts. If you are wanting to batch delete please click on the help bottom on the bottom right corner and ask us to turn in the 24 hour "Batch Archive". You will be able to select all the contacts you want and batch archive them.