Hi all!
My openSUSE version is 11.0, and I want to update it to 11.1 . Laptop that I use have 20gb of HD, so I can’t update it from DVD installation. And the question is how safe is to update whole system from NET installation? 
It’s not entirely clear what you mean.
The DVD will happily install to a 20 GB hard drive. 10 GB is probably plenty for a root partition. The .net ISO doesn’t install any smaller than the DVD - it just downloads packages at install time, rather than putting them onto the DVD first. (In fact, if anything, the DVD might give you more control over what you install, and therefore let you install to a smaller drive - I’m not sure.)
You can’t easily install 11.1 on another partition while still using 11.0 in 20 GB, and I assume that’s what you mean, and when you say NET you sound like you might be referring to an online upgrade - as in updating SUSE from a running system.
In which case it is reckoned quite reliable, though certainly not foolproof. Remove any extra repositories you’ve added, and back up all your data first, then give it a shot. If it goes horribly wrong… Well, you were trying to reinstall anyway, weren’t you?
Thanks. I thought of .net ISO installation, so I didn’t knew how much load it drags from the internet… too much you say 
Well I’m gonna stay with my 11.0 version, it’s totally fine and still pretty fresh release