To uninstall Tails from a USB stick, and use it for something else, you have to reset it.

The content of the USB stick will be lost in the operation, but an attacker might still be able to tell that Tails was installed on that USB stick using data recovery techniques unless you securely clean all the available disk space afterwards.

The following instructions do not work on Windows XP.
The version of Diskpart on Windows XP does not list removable disks.

Using Diskpart

You might overwrite any hard disk on the computer.
If at some point you are not sure about the disk number, stop proceeding.
  1. Make sure that the USB stick that you want to reset is unplugged.

  2. Click on the Start button, and choose All Programs ▸ Accessories ▸ Command Prompt, to open the Command Prompt,

    More help on how to start the Command Prompt

  3. Execute the diskpart command, to start Diskpart.

  4. Execute the list disk command to obtain information about each disk in the computer.

    For example:

    Diskpart> list disk
    
      Disk ###  Status      Size     Free     Dyn  Gpt
      --------  ----------  -------  -------  ---  ---
      Disk 0    Online        80 GB      0 B
    
  5. Plug the USB stick that you want to reset. Run the list disk command again.

    A new disk, which corresponds to that USB stick, appears in the list.

    For example:

     Diskpart> list disk
    
       Disk ###  Status      Size     Free     Dyn  Gpt
       --------  ----------  -------  -------  ---  ---
       Disk 0    Online        80 GB      0 B
       Disk 1    Online         4 GB      0 B
    

    Make sure that its size corresponds to the size of the USB stick that you want to reset. Note down the disk number assigned by Diskpart to the USB stick.

  6. To select the USB stick, execute the following command: select disk=number. Replace number by the disk number of the USB stick that you want to reset.

  7. Execute the clean command to delete the partition table from the USB stick.

  8. Execute the convert mbr command to create a new partition table on the USB stick.

  9. Execute the create partition primary command to create a new primary partition on the USB stick.

Troubleshooting

See the Diskpart documentation from Microsoft Support.