How to change the default selection of the checkboxes?

Frequently Bought Together allows you to control which recommended products start pre-selected, so you can decide how the offer is presented to your customers. To do so, please open the app preferences page and navigate to Visual styles → Checkbox.

There you will find an option to customize the default selection, as shown below:

All products selected — Every recommended product starts selected, maximizing the initial order value.
Only the first product selected — Only the first recommended product starts selected; customers add the rest manually.
All except the first product selected — Every recommended product starts selected except the first one, which customers add manually.
No products selected — No recommended products start selected; customers choose everything themselves.

Tip: pre-selecting products increases the chance they're included in the order, while leaving them deselected makes the offer feel less pushy and more like a genuine suggestion.

Lastly, please remember to click on Save, so that the changes can be successfully applied.