To do it is very simple.
First of all, click on the name of the Block you want to modify that you find in the left sidebar
Then click Content Structure, the second item on the tab
You will be redirected to the page that allows you to edit the content model. In the left part of the page you can modify the model by going to remove, add or modify each Element. In the right column, however, you can change the configuration of your Block, such as the Name, Description, Icon, Features, etc ...
Once the modification is complete, click on the Save button.