There are 3 ways to delete a topic - only one of which is even remotely obvious - sorry about that!

Technique 1

Tap and hold a topic until the link with its parent is severed (about 2 seconds) Please note - you need a steady hand - any movement and it will not work. Remove your finger and you will be presented with the delete option as below:

Technique 2

Double tap the topic as though you wish to edit it. Scroll to the bottom and select ‘Remove Topic’ as below.

Technique 3

Tap an already selected topic and select CUT. This has the advantage that if you change your mind, you can simply paste it back.