Thanks for the answers. I didn't formulate the question right. Let me try again.
I used to own the number, when I traveled abroad. While I owned the number, I connected the What's App and had many chats. I did a backup of my chats on my google drive. I can see the backup in my drive.
When trying to access the backup, it asks me to connect to the phone number associated with the backup. That number doesn't belong to me anymore. I can't access it I can't because the number was given to someone else.
My question should have been: How do I get access to the backup (which is my google drive folder) without having to re-access the old phone number, which I don't own anymore. Sorry for the confusion earlier. Not trying to hack into someone's phone.