If you click on, for example, ‘Asset Fields’ (A above), the existing asset fields will be displayed. You can modify existing ones by double clicking on them. Press <Return> key to save changes.
To create a new one, double-click on the box that says ‘Double click the field to input text.’ And enter the new asset field. Press <Return> key to save.
You cannot delete an item but you can archive it.
Tick the item(s) you wish to archive e.g. ‘Asset 2’ and ‘Asset 3’ (A above) and then click on the ‘Archive selected’ button (B above). The item(s) are then moved onto the ‘Archive’ column on the RHS. E.g. ‘Asset 2’ and ‘Asset 3’ have now moved to the Archive List (C below).