There are two ways: tick the round check box at the right of the row in the list, or open the transaction and click Mark as reviewed (the button changes to Reviewed), then Save Transaction. Reviewing means you have confirmed the category. Effects:

• The row stops being bold in the list.
• The Auto badge disappears.
• Sprwt creates or strengthens a Learned Rule for that description and category so similar future transactions are categorized automatically.
• Re-categorize All skips reviewed transactions.

Untick the box or click the button again to set it back to unreviewed.