How do I link a booking to a different customer?

Open the dashboard of the relevant booking. In the ‘Customer details’ block, click on the gears icon that appears when you hover the mouse over the block title (dark grey line). In this dropdown menu, click on 'Link to other customer’. A search window opens. Search on the name of the relevant customer and click on the line (not the id). That customer is now linked to the booking and the ‘Customer details’ block is now populated with that customers personal details.