There are several ways to remove the write protected status on a flash drive, depending on how it was write protected.
If a file is read only, not the whole flash drive, you can go to the files properties, and uncheck the read only checkbox.
If a flash drive has a small write protect switch on the side that may have been flipped inadvertantly, switch it back to unprotected. Here is an image of such a flash drive - some of them have a swtich, some don't.
Lastly, if the flash drive was software write protected, you need the program used to do that, to unprotect it.