You can download the latest version of Magento and then take everything from app/code/core/Mage/Authorizenet. You will need to review what other files the extension uses, e.g. app/etc/modules, skin/ js/ app/design/frontend/base/default/layout and app/design/frontend/base/default/template.
In general, the recommendation would be to upgrade the entire Magento code base.
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