Update Cascade version3.0.0

I see today in my shop that there is a new version of theme Cascade. I should update to version 3.0.0. I’m reading in the Shopify guide and information that during the update I could lose the modifies I made in the theme’s code. So I’m very worried to lose everything and wake up after the update whit a shop completly different and weak.

Could you help me? Which information I will lose?

Thank you all,