For you to be able to re-write a DVD, it must be a rewriteable DVD, also referred to as:
- DVD-RW
- DVD+RW
- DVD-RAM
which are different standards for rewriteable DVD-media. You can read more about the formats here
What you're probably experiencing, is that your DVD media is a non-rewriteable media, which are called:
- DVD-R
- DVD+R
You will need to confirm which type of disc you have. If it's one of the 3 types in the top list, you need to go into your disc writing software of choice and erase the disc. If you dont have disc writing software, you could use (for example) CDburnerXP, which is a Free utility, which runs on all modern versions of Windows.
When you have erased your disc, you can write your new ISO file to the disc and start using it.