You can edit text in the Score Editor. Select the arrow tool in the toolbar (the first one on the left). Make sure the T is selected in the object palette. Select a block of text by double-clicking on the top left corner of the text. The Edit menu will appear. Choose the text editing operation you would like.
You can also erase or move text using the other tools in the toolbar (except the pencil, which is used for entering text). Make sure the T is selected in the object palette. Select the tool you want and apply the edit by clicking on the beginning of the block of text (to erase using the eraser tool) or by dragging the block of text up, down, left or right.