An entity is defined by its attributes. Each entity type has a different set of attributes. Object attributes, for example, can be type, position or health, while the marker is configured by size or color values.
Entity attributes can be tweaked in the Edit Attributes window. To open it, double-click on the entity.
Alternatively, you can click the right mouse button on an entity and pick Attributes option from the context menu. When multiple entities are selected, attributes will be modified for all of them. This is useful for batch changes, e.g., making multiple characters playable in multiplayer.
Attributes in the Edit Attributes window are sorted into categories, each of which can be expanded and collapsed by clicking on their titles. Their state is remembered and restored after returning.
The description for each attribute is available in a tooltip when hovering over the attribute's title. The modified values are only saved after pressing OK; clicking on Cancel will discard all changes.
When multiple entities are being edited, an attribute can only be edited if its value is shared by all of them. When it is not, the attribute is disabled and the original values will not be overwritten. If you wish to edit the attribute, enable it using the checkbox on its right. The modified value will be applied to all the selected entities.
Some entities, in particular modules and other systems, may have specific attributes unique only to them. They are always shown as the last category called Entity Specific - [Entity Name].
Attributes are a cornerstone of the editing process. Also, be sure to check the Eden Editor: Scenario Attributes tutorial which will explain how to configure the whole scenario, not only the single entities.