When opening an Overview you will se the default columns with data related to the entity, that the Overview type was made from. Usually these are the Organisation name, and depending on the type of Overview, decriptions on Pipelines, Revenue on Orders, Responsible etc.

To see more data - or to choose a specific view of columns - click on the column icon in the right side of til picture.

From here you can search the field you want to show in a column, together with unmarking / deleting an existing column.