New mac but want to use time machine and erase old mac files
I have a new mac and want to use time machine. I have an external drive that I used for the old mac. I want to use that external drive for my new mac. I chose not to inherit old backups when I first used time machine for new machine and now I think it's keeping the backups for the old machine because there's not enough space for a backup now. I don't want to keep the old backups from the old machine. Do I have to reformat the external drive or something? If the answer is yes, please give instructions. If it's something else, please also give instructions. Much appreciated.
MacBook Air 13″, macOS 10.15