The first questions here would be to clarify what you mean by "retire" the Time Capsule as far as what you want to do with the data that is currently stored on the TC disk.
Do you want to leave it there?
Do you want to erase the data on the disk?
The next questions would be what you want to do with the stored network settings on the TC.
Do you want to leave them "as is"?
Do you want to erase the network settings and reset the TC back to its original default settings?
Once you decide what you want to do with the data on the TC disk and network settings and have completed the tasks, you would disconnect the TC from the network by powering off the TC and removing any cables that might have been connected to the TC.
Unfortunately, we probably cannot provide much help for a Seagate product on a support forum for Apple products, but if you could provide the model number of the Seagate hard drive, that might provide some clues that would give us a better idea on how to use Time Machine on your Mac to back up the data on the Mac to the Seagate hard drive.