Changing Microchip Info

Contact the outfit that registered the chip. There are quite a few of them, but since the number is on AKC papers, AKC should have that info. They have a program called AKC Reunite. So you contact the keeper of the registration database for your dog's chip and submit proof of transfer of ownership to them. The paperwork you have should do it.

I don't have any idea who registered the chip. All I have is a number on the AKC papers. I lost contact with the previous owners after they moved back to Germany.

You can contact AKC Reunite here to see if your pup's microchip is on record with them: https://www.akcreunite.org/contact/

If so, they have a transfer process they can walk you through to update the information.

Otherwise, your vet may be able to tell what brand/company microchip you have when they scan it. If you really can't figure out what company the microchip belongs to, it is possible to insert a new one safely (my mini poodle has two due to his first not being compatible with international standards), but this would be a last resort because it could cause confusion to have two microchips registered to different people.

I know that for the main microchip company I use, I am the one who has to start the transfer process. Other companies might have different policies. (Both of my dogs are also registered with AKC Reunite, but only because they were already chipped when I sent in their PL and CP applications.)

When you got the dog, they should have done a transfer of ownership by filling out and signing the back of his registration certificate, and you needed to have sent in his certificate and the transfer fee to get him registered in your name.

When we adopted an adult dog who had been microchipped, we were able to get his number from a vet. A vet should be able to tell you which company the chip is from.
Just contact them and tell them what's going on. We had to provide proof of ownership and there was a small fee (around $20). They updated everything with no problems.