Depending on what data feed app you are using, if you are using the free app, you can not change it, unless you change the image in your Shopify account.
There are some apps that allow you to choose a different main image for Google Merchant Center.
However, the easiest way would be to create a supplement feed and assign your own images. Here is a guide: https://support.google.com/merchants/answer/7439058?hl=en
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