Question: TimeMachine backups
I've just used my TimeMachine to migrate the backed up data from my 21.5" iMac, to my new 27" iMac. I want to continue using it to back up my new computer, but it says there is insufficient space to do so. How do I get my new iMac to over-write the older backups and continue as before? If formatting it, and beginning again is the best way, please describe how I go about it. All the necessary data, has been transferred, and I don't need any of the old backups.
iMac with Retina 5K display, macOS High Sierra (10.13.1), 3.8 GHz Intel Core i5, 8 GB DDR4