To type text, just press the letters on your keyboard and those letters will be inserted at the location of the caret in the script.
You can also edit it overwrite mode. (You have been editing in insert mode.) In overwrite mode the characters you type overwrite the character the caret is on rather than inserting themselves in front of the existing characters.
When you go to overwrite mode the caret will become a block covering all of the letter that will be replaced. Also in the status bar, the text INS will be replaced with OVR. Overwrite mode will not overwrite an end of a paragraph. You need to press Delete at the end of a paragraph to join two paragraphs, even in overwrite mode.