There are a couple of ways to back-up your Shopify Store:

1. You can manually back-up your store data by exporting a CSV of your Product
details (Products > Export) and you can download your Theme File to your 
desktop device (Online Store > Themes > Actions > Download File). We also have
a guide to this process HERE.

2. There are apps available which can automate this process. 
Rewind (https://apps.shopify.com/backup?utm_medium=Forum&utm_campaign=Guru+Ask&utm_source=social)
and 
Replay (https://apps.shopify.com/store-copy?utm_medium=Forum&utm_campaign=Guru+Ask&utm_source=social)
are two examples.