Text which was there before you entered input mode will not be changed. It is possible, however, to change existing text for new text using the c command. The c command is a combination of delete and insert. Move the cursor to the beginning of the text you wish to change. Type cw to change the current word from the cursor to the end.
Type ce to change the current word from beginning of the word to the cursor.
If you do not enter new text, the c command is equivalent to the d command. If you delete text from one location, you can insert it into another. This will insert or paste the buffer in the new location. Perhaps you would like to paste a segment of text without altering the existing text. To do this use the yank operator.