Backing up a backup?

I had someone in to set the existing Seagate up and as far as I know it backs up automatically and I never write anything to it. As you can see I am not technically proficient. I thought that if I just bought another 1T Seagate all I needed to do was to unplug the existing one and plug the other one in its place or is this simplistically nonsensical?

It backs up automatically. So your PC writes to it automatically in the background. Then it certainly is a bad idea to replace the external while the system is running.

It might very well be possible to do it when the system is shut down, but that might depend on how that background program works and where administration it keeps where. I'd ask that someone that set it up for advice.
Also, having a backup is fine. Having 2 backups is even better, but it might complicate getting things back from the backup when needed (and that's what a backup is for, isn't it). That's also something to discuss with that tech. It would really be a pity if you have 2 external disks and don't know how to use them to get the most recent version of your files back.

My excuse for technical incompetence is that I am 80 plus obviously not one of the generations that were bought up with computers and who think like them intuitively. At my age obviously I came to computing late in life. I have asked Seagate to comment on my idea and await their reply. I would have hoped that if I bought a mirror image of the device that I have I could wait until I close down one evening and just switch the devices who I would assume would just carry on backing up but with one in a draw just in case Ransomware nobbled the other.
The whole point is that surely there can be no point in having a backup if it is accessible to Ransomware and a valid backup must be inaccessible such as in my desk when I change it weekly or so.