I can delete a contact and a group but can i delete a topic as group owner

Yes, you can.

As long as you are the group owner, you can edit topic titles and delete topics. Look for the buttons to do that under the topic title

I think the topic creator can edit the topic but can't delete it.

So there is no way to delete a topic if you are not the group owner??

You could ask the forum owner very nicely to delete it for you... or you can still edit the topic's title, to reflect something different if you wish.