Select the block of text:
1. Position the cursor at the beginning of the text you want to replace
2. Press v to enter visual mode. (or upper case V to select whole lines, or Ctrl-v for a vertical block).
3. Move the cursor to the end of the text to be replaced. (While selecting text, you can perform searches and other advanced movement, a feature that sets vim apart from most other editors.)
4. Press c. This deletes the selection and leaves you in insert mode.
5. Type your replacement text.