In Umbrello UML Modeller, there are basically two ways for editing the elements in your model.

Using the context menu of the different items in the Tree View you are able to add, remove, and modify almost all the elements in your model. Right clicking on the folders in the Tree View will give you options for creating the different types of diagrams as well as, depending on whether the folder is a Use Case View or a Logical View, Actors, Use Cases, Classes, etc.

Once you have added elements to your model you can also edit an element by accessing its properties dialog, which you find by selecting the option Properties from the context menu shown when right clicking on the items in the Tree View.

You can also edit your model by creating or modifying elements through diagrams. More details on how to do this are given in the following sections.