iMac not allowing delete files on Seagate portable 2TB hard drive

My late 2017 iMac running Mojave 10.14.5, is not allowing me to delete certain files that I have on my Seagate 2TB hard drive, that was originally formatted to my windows desktop, before I switched to Mac. So I am too unfamiliar w/the software, to know what I am doing wrong. When I bring up permissions for the Seagate, it says I, the admin., and only user, has read and write permission, but staff, and everyone, read only permission. When I put in the admin. password, and unlock it, and change the other 2 to read and write, it says," I am not authorized, to make this change". I do not even know if this would have solved my problem or not, since the admin DOES have read and write permission.

I thought about reformatting the HD, to the iMac, but quickly realized I will lose, almost 2TB of important data. And I really need to remove these files, that accidently were written to this HD, if anyone can help. Thank you.