Question:
We would like to change our payment gateway to Authorize.net , How would we do that in Shopify.
Reply:
This is fairly easy to do by going to the Payment area of the Settings section.
If you have a gateway already set up or it came set up with the default Shopify one you have to deactivate the old one before adding the new one.
Here are some nice directions:
https://help.shopify.com/manual/payment-settings/third-party-gateways/set-up-authorize-net