I have already installed Elementary Os. But when i want to format the USB the files are protected and i cannot delete them. Thanks for your answers.
SilverWolf is correct. As long as Elementary OS is installed on your hard drive (or SSD) you can safely reformat your installation USB drive.
One way to do this is through GNOME Disks
a. Go to AppCenter, search for GNOME Disks.
b. After GNOME Disks has installed go to Applications -> Disk
c. You will see your drive list on the left column.
d. Select your USB disk (make sure you choose the correct disk).
e. In the middle column, click on the stop icon (square).
This next step will clear all of the data on the USB disk. Be sure you want to proceed before continuing.
g. Click on the minus icon to the right of the stop icon.
h. Press the Delete button to delete the partition.
i. Press the plus icon, then click the Next button to continue.
j. Enter the volume name and leave the type as FAT.
k. Now click the Create button.
NOTE: With a USB disk you do not need to erase the media.
l. Finally, click the play icon to remount the device so it can be used by Elementary OS.
Your USB disk will now be blank and ready for use.