If you want to update your phone number for the verification process at checkout. First, you need to opt out from the process by visiting https://pay.shopify.com/ , in the bottom of the page, either enter your email or the number that you wish to replace it with a different number.
Once you're opted out, the next time you get to checkout, just enter a new number as if you're doing it for the first time. Where you enter the number would look like this.
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