I have an older Mac desktop and an external hard drive that is formatted for PC with files on it. I want to keep those files but format it for Mac also. Any advice of how to do this is appreciated.
My concern is that I do not want to lose the files that are already on the hard drive.
When you format a drive, you are erasing it and then starting fresh in a specific format. There is no way to change the format and keep the files. You would need to copy the files elsewhere, be very sure you have those files safe and sound, reformat the drive, and then copy the files back.
The drive is either formatted for Mac (APFS or HFS) or Windows (FAT, ExFAT, NTFS). It can't be formatted two things at once.
However, Macs can read all Windows-formatted drives, and write to some (FAT, ExFAT). So if you really need to have a drive that can be accessed by both, then use ExFAT. But I'd need to understand what you are really trying to DO here. Today it usually isn't necessary for two different computers to share a single drive like that. Cloud services or network file sharing is usually the better option.