if your a package update is not expired, when you update it to c package, you will get one year update for c package
If your a package is expired already, you need pay update subscription for a package firstly, then update it to c package

If you want to update the a package to c package full version, you need pay for this: https://www.eobdtool.co.uk/wholesale/update-obdstar-x300-dp-plus-a-to-c-package-full-version.html
If you just want to buy update subscription service for A package, you need pay for this: https://www.eobdtool.co.uk/wholesale/obdstar-x300-dp-plus-update-1-year-subscription.html