How to upgrade Ubuntu release on a computer without internet connection?

Please use the Ubuntu alternate cd to upgrade, simply put the cd into driver and Ubuntu ask you to upgrade...

A suggestion: install Deluge client, to download torrent files, from http://deluge-torrent.org/downloads.php and please download the Ubuntu 8.10 release you need by selecting a .torrent link from http://releases.ubuntu.com/releases/8.10/

Here all the Ubuntu releases http://cdimage.ubuntu.com/releases/ with the alternate images .torrent links

Also please be sure to check and compare the md5sums of your downloaded iso images and burned cd or dvd with the MD5SUMS link contents, provided by Ubuntu, you can find it into the same download directory
Here an howto https://help.ubuntu.com/community/HowToMD5SUM

Here the Ubuntu world wide mirror lists http://www.ubuntu.com/getubuntu/downloadmirrors

Please burn the downloaded alternate .iso images on a cd at lower speed you can do (4x)
Here there is the official burning howto https://help.ubuntu.com/community/BurningIsoHowto

To upgrade Ubuntu 8.04 Long Time Support release to Ubuntu 8.10 you might need to edit the:
System→Administration→Software Sources→ [ Updates tab ]
Release upgrade: Show new distribution release, select "Normal release" from the list box.

Always useful is the online Ubuntu doc https://help.ubuntu.com/8.10/