Two includes a child theme to make the updates easier.
If you are planning to do several CSS changes, it is highly recommendable to do these changes inside a Child Theme, so you will never lose these changes.
The steps are pretty simple:
- Upload wp-two-child.zip into wp-admin ⇒ Appearance.
- Activate Two Child.
- You can now edit any file inside Two Child, such as style.css or functions.php.
Important: make sure that both Two and Two Child are uploaded, and Two Child is the one activated.
Note: you can use the Appearance ⇒ Theme Setup to automatically install and active the Two Child theme.