To put stuff on it, just drag it to the icon of the Flash Drive on the desktop.
To remove stuff, drag it to the trash and empty it.
In the scenario that you described, I'm not sure if you have the original on the Mac and are copying the original onto the Flash Drive or whether the original is on the Flash Drive and you are opening the file from the Flash Drive.
If the first case, just change the original on your Mac and save it. Given that the name of the file is probably still the same, drag the original onto the icon of the Flash Drive and it will, after confirming with you, replace the out of date file with the new one.
If the second case, change the file and save it. As the file was originally on the Flash Drive, you need do nothing else.
No need to format the stick at all
Hope that helps
1st. time user of a flash drive. My ? is...
When you add a particular file, and later the information changes on that file, do you just drag it to trash and replace it with the new version, or must you re-format the
entire flash drive and start over?