How to delete Time Machine backups from an usb external drive?

I have used 2 different external usb hard drive to do time machine backup (on demand backup meaning whenever I needed it; not a regular basis) alternatively.  So the backups are in date sequence. For example: July backup in one drive and August backup in another drive. Now I need one of those drive to store other files. Is there a way to move/copy the backups from one of the drive to the other? If so how to do it? The backups on both drives are from 2011 MacBook Air with Sierra & HighSierra.