Finder refuses to let me edit my USB

How is the usb drive formatted? If it is formatted in the windows format, ntfs, os x can only read from that format and can not write to it. So since it can not write to that format you can't change, delete or add any files to the drive.

To be able to fully utilize it on a mac computer it must be formatted fat32 or exfat, a windows format that both windows and os x can read and write to, or of course the mac format, mac extended (journaled) with a partition table of guid.

To do that you use disk utility. But if you are going to format it exfat it is best to do that on a windows computer.

An alternative to reformatting the USB drive (which would erase the data) would be to purchase and install one of the third-party NTFS drivers available on the market. I use the one from Paragon Software.