If I understood you right you want to delete data on you Sandisk usb stick.
To do this follow these steps:
1. Insert the usb stick in your machine
2. Enter "Computer"
3. Your USB device should be showing as an alternative harddrive with a specific letter (Usually C, D, E, G")
4. Right click the usb device, and choose "Format"
5. Press "Start" and wait abit.