how to upgrade ubuntu 8.10 to 11.10

You can not upgrade to 11.10 from 8.10 . It is not supported. only from 11.04 you can upgrade to 11.10. If possible try backup your data and do a clean install for 11.10.

Think twice before going to 11.10 after you have been using 8.10. You will have ONLY the Unity interface.
The Unity interface is a nuisance to use, and takes more time to open your programs.
I suggest upgrading to 11.04. That version allows you to use either the Unity or the Gnome interface. There is a box on the lower panel of the sign on screen that allows you to select 'Ubuntu' ( Unity interface ) or 'Ubuntu Classic' (Gnome interface).
I upgraded from a working 11.04 to a buggy 11.10. I have used 11.10 for about 3 weeks and HATE it. After I document a few bugs, I will load 10.04LTS or 11.04, and use them as long as I can.
A few bugs:
     The black and white theme is very hard to read.
     Each time you log on it will move the icons on your desktop.
     The 'launcher' constantly gets in the way of using controls (back arrow on firefox) on the left of the screen.
     The icons on the launcher are 9 times the size of the icons on the old panel.
     The 'dash' may show the item you are looking for, but it also shows a dozen more you have no use for.
     The 'dash' will randomly show icons of files you have recently accessed or Internet sites you have visited to anyone looking at your screen.
     Movie player and banshee start playing, then freeze, then start playing after about 30 sec.
     Image viewer will not show more than about 4 images before freezing.
     In general everything is slower, harder to open, and harder to see with 'Unity'.

Some people like or dislike Unity. I will not discuss it. You should load both version, make both CDs and make your opinion by trying them (boot on CD, and choose "Try without install").
You can't upgrade from 8.10, and even if you could, I will not advise it because there are two many steps and it's quite sure you will fail.

One process is:

1-make a backup of your home folder (and of valuable data outside home folder if any)
2-burn a CD of a new Ubuntu (11.04 or 11.10) or make a bootable USB key
3-boot on Ubuntu medium
4-select manual partitioning
5-make 3 partitions
5.a-a swap one of 2GB (or at least the RAM size to allow hibernation)
5.b-a 10GB ext4 mounted on /
5.c-with remaining size an ext4 mounted on /home
6-continue installation, with this empty /home folder
7-check new installation boot and works
8-boot again on Ubuntu medium
9-choose "Try without install"
10-plug backup disk and copy /home folder from backup to internal disk
11-reboot
This process, which give you an independent home folder, will make easier future installation.

Another process, more simple, is:
1-make a backup of your home folder (and of valuable data outside home folder if any)
2-burn a CD of a new Ubuntu (11.04 or 11.10) or make a bootable USB key
3-boot on Ubuntu medium
4-choose manual partitioning
5-mount folder "/" on your current ext2 partition and DON'T FORMAT
6-continue installation
7-check /home folder was not corrupted
In theory, since 10.10, Ubiquity (Ubuntu installer) is presumed not altering /home folder. But please, make step (a) even if it will not probably be useful.