how do i download from my seagate backup to my new imac?

First, a question: what "driver"? External hard drives do not need a "driver" or any other software to function.

If you have a Time Machine backup on an external, simply plug it into your new Mac and, when first setting it up, use the Setup Assistant offered during initial set up and everything will be transferred. You can use Migration Assistant later; however, that can result in some problems because you may wind up with two user accounts.

Is the External Hard Disk a Time Machine backup? If so, connect it using a USB caable to the new Mac and launch Migration Assistant and let MA do its job.