There are two ways to edit text in the block:
1. Some blocks may be edited using a WYSIWYG (What you see is what you get) editor where text can be edited as soon as toggling the "edit' button. You will see an orange dotted outline when this is active. Simply click the text box and change the text then save.
2. For other blocks or sections of the blocks, you'd need to click the "Pencil" icon at the top leftmost portion of the block. An editing page will pop out of the left of the screen and in the "Content" tab you'll find the editable text.