Question about Ubuntu/Debian!

Now, first of all, I'm not so much of a hacker you might say, but I am USUALLY very good with computers. So DON'T flame me, I already know im the biggest noob here. I just have a very important question.

I have Linux on my harddisk on my computer. (ubuntu) I want to put Debian on my harddisk as well, but I don't want ubuntu on there anymore cause it will take up too much space. How do I write over existing Linux OS's?

Thanks for any help ahead of time :)

same way you would do with other os'es, during install of debian just pick your hd as / where your previous ubuntu install is on.

You must Format your hard drive ;)

Yeah, the debian graphical installer specifically gives you an option to overwrite existing partitions/drives.

I have been doing with Debian for a while, as server OS. I used the netinstaller cd, anyway. When I do just a little bit mis configuration, I am afraid about the security, so I start the installation again, to a fresh start (thats just how I am xD). What I noticed when I was going to install Debian over Debian was that I got an Grub error xx (17 or something, idk). Because of the MBR (Master Boot Record) in the computer was told to boot up Debian(1) that dint exist, and Debian(2) wich existed. The problem caused because of the partitions. So what I did to resolve this was easly get my windows xp cd (dint install windows) when trough the start, but when you are going to choose HDD, you can delete partitions. So I deleted all partitions on my HDD, quited (F3-key). And then started install of Debian, again. Then it went OK. Since the MBR dint crash with any other partition. Only one to start up this time.

I hope you folks understand, might be abit hard if you havnt been there before. And I am sure that ther is other ways to clear the partitions, but that was the way I got trough.

Youst a remider, or notice, if something like this is happning.