Shopify theme updates have always been a real trouble for the store owners. We have prepared a video tutorial for you on how to upgrade Shopify theme in a most convenient and stress-free way.
Of course, never publish and THEN update theme, but take unpublished theme and use preview mode to check if the new theme looks exactly how you want before publishing.
OLD QRACK THEME CUSTOMERS: You will need to manually setup the store from scratch if you are upgrading from themes prior to v2.0.0. No easy upgrade available. For any updates after v2.0.0 you can use the tutorial below.