How to pick and select items from the Asset report box during Level migration?

Hi there. When I try to migrate a level to another project, the Asset Report box presented is already pre-filled with all the assets from the level and I am not able to pick and select anything, as referred in the level migration documentation. Can I get some help on how to select items on the level migration asset report box, as I only want to migrate part of the level content (other part is updated in the destination project and I don’t want to overlap with older versions of some assets). Many thanks Paula