Why don't you have an internet connection? Is this just a temporary issue?
Is it a laptop, could you bring it into work and update there?
Another option is to share your connection from your phone, if you are able. I had to do that once with a minor update as my WiFi wasn't working.
Another option would be to copy the installation files from another Mac and install them at home. Here's how to make a bootable instal drive: click here, I think Apple offers the OS on a drive for enterprise use, because it's easier to do the install that way, but I'm not sure they would do the same for a normal consumer, and I don't know what process you would have to go through to get that.
I think you need to take your Mac to somewhere where you can get online if you can't at home!