See this link and links from it for details about booting from USB,
Installation/FromUSBStick#Booting_the_Computer_from_USB
-o-
There are tools for installing an iso file into a USB drive in order to make it a USB boot drive with Ubuntu. But you should not have any other file on it, at least not when you make it a USB boot drive. So please copy the file(s) from the drive to some other drive, using another computer, if you have no working computer now.
Cloned live-only system
Do you intend to use this USB drive only during a short period of time in order to install Xubuntu into your computer? In this case I suggest that you make a simple cloned system. It is read-only and the drive cannot be used for anything else, but can be restored to a storage device afterwards.
In Ubuntu ('family' systems including Xubuntu) version 16.04 LTS and newer you can use the Ubuntu Startup Disk Creator alias usb-creator-gtk. In older Ubuntu versions you can use Disks alias gnome-disks or mkusb. mkusb can also restore the pendrive to a storage device afterwards, via the wipe menu.
In Windows you can use Win32 Disk Imager for the same purpose.
Persistent live system
If you wish to have a USB boot drive, that you want to tweak, and also use to store and transfer files, you can create a persistent live drive. mkusb can create such a drive too. See this link: mkusb/persistent