I wish to unistall Dawngaurd on my Xbox because it has a bug which will not allow the alchemy lab in Whiterun to be purchased.

However I am not sure how to do this without deleting it from the memory.

So does anyone know a way in which I can temporarily unistall it ,buy the alchemy lab, and then reinstall Dawngaurd?

You can simply uninstall it from the hard drive, buy the alchemy table, then download it again for free.