They provide flow arrow, you can edit them to marking the direction.
To add "Above Label" or "Below Label", you need to select 2 topics at firs.
- Click on a topic.
- Press "Ctrl" key, and click on another topic.
OneNote will select 2 topics.
After select 2 topics, click "MindMap" tab -> "Add Topics" group -> "Label" to add a little label above the connector between these 2 topics.
← Label →
Edit the label, remove ←, to making the direction to right.