1. In your Shopify Admin go to online store > themes > actions > edit code
2. In your theme.liquid file, find the <head> (press CTRL + F or command + F on Mac)
3. paste this code right below the <head> tag:
<!-- Leveraging Browser Caching --> <meta http-equiv="Expires" content="30" />
The meta data above means that document is considered expired after the specified date and time. If you set it to “0“, the browser will check for a fresh new document on each visit.
I reckon that 30 days is good enough.
Keep in mind that it may take a few days to update.