cannot upgrade to 8.10 from 8.04

Go to Accesories->Disk Usage Analyzer

Click on Edit->Preferences

this will show which partions you have and where '/' is mounted (only check that partition, uncheck the others)

then click on scan filesystem to see what takes space

if you haven't done this yet empty your trash

Thanks Koen for your suggestion. But my problem starts from there; since I was already trying to free space from "/". My home has lots of space in it; but apparently that is not enough!! It probably needs space in folders such as /lib or or /usr but I have no idea which packages to delete; so that it wont cause any problems in my system.

When I use disk usage analyzer, it gives the following stats:

Syst capacity: 7.5 GB used:6.4 available:1.1
/ 100% full
breakup:
/usr 82% full (5.0 GB)
    /usr/share: 35.4% (1.8 GB)
    /usr/lib: 28.3% (1.4 GB)
    /usr/local: 23.9% (1.2 GB)
    /usr/src: 7.5% (384.4 MB)
    ......
/lib 11.6% (746.2 MB)
   /lib/modules 62.1% (463.3 MB)
  /lib/linux-restricted-modules 30.9% (230.2MB)

My trash is already empty. Please advise me which files I can delete.

Further information to help you. When i do df -m, I get the following output. Why is /dev/sda6 so full? It shows that it is not a directory!Please help.

XXXX@XXXXXXXX: df -m
Filesystem 1M-blocks Used Available Use% Mounted on
/dev/sda6 7690 6570 730 91% /
varrun 1013 1 1013 1% /var/run
varlock 1013 0 1013 0% /var/lock
udev 1013 1 1013 1% /dev
devshm 1013 1 1013 1% /dev/shm
lrm 1013 39 974 4% /lib/modules/2.6.24-22-generic/volatile
/dev/sda8 17890 890 16092 6% /home
/dev/sda2 10240 3198 7043 32% /media/sda2
/dev/sda3 71689 28921 42769 41% /media/sda3
/dev/sda5 2551 754 1798 30% /media/sda5

Well, when i ran out of spacei run---- sudo apt-get autoremove. it worked for me. Have you tried installing system-cleaner-gtk(Cruft remover) from the synaptic package manager.
when you finish installing, run from the system...........> Administration........> Cruft remover.
Cruft Remover will clean unwanted files from your system, but be careful.

hopes this gives you an idea or helps pehaps.

cheers.

I am still at a loss.
FajardJorg, I tried your suggestions and this is what it gives:

$ sudo apt-get autoremove
Reading package lists... Done
Building dependency tree
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

$ sudo apt-get autoclean
Reading package lists... Done
Building dependency tree
Reading state information... Done

$ sudo apt-get install system-cleaner-gtk
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package system-cleaner-gtk

I have made sure that I have main, universe, multiverse and restricted software repositories enabled in Software sources package. Help!!

I upgraded using the alternate cd. It solved my problem. thanks.