If you do not have a separate /home partition and it sounds like you don't
or you would not be asking, then this is a lesson on why many users opt for
one. With a separate /home you would just re-use it and not worry about
losing data. If you have enough hard disk space then it is worth the time to
do a fresh installation and set one up. However, that is not the problem at
hand.

You need to save your entire home folder to another drive, usb key or
partition, just in case. Make sure that you the the hidden folders too
(Ctrl-H) as that is where your settings are. You can do some housekeeping
and delete caches and cruft first if space is tight.

Then you have a choice. You can do an upgrade or fresh installation. Try the
upgrade first. http://www.ubuntu.com/desktop/get-ubuntu/upgrade

If that does not work then then opt for a fresh installation.

After either, copy your old home data back to the new home folder replacing
what is there.

If upgrades are worrisome then stop there and do not upgrade to 10.10 that
way you will have some consistency for two and a half more years as 10.04 is
LTS. Besides Ubuntu is in for a transition with 11.04 and you can avoid that
and see how things shape up. I am excited about the change, but many do not
like radical change.