I recently needed to update a clients Magento installation from version 1.4 to the latest 1.5, now unfortunately this isnt as easy as with other Open Source Applications (the WordPress update feature is as easy as clicking your mouse a couple of times) and there is a lack of easy to find documentation on how to update Magento.
Now if your happy with the way your site is running then you dont have anything to worry about, just keep using 1.4, however if you want to take advantage of a feature in 1.5 or there is a new add on which wont work without it then you may, like I was, be sitting there scratching your head for a while.
Numerous users have talked about attempts to update via Magento Connect with no joy which left only a few options, I choose to do the update using something called Secure Shell, or SSH, which works with command line prompts so users of DOS or Unix/Linux will feel right at home.
These steps are based on the posts by Magento forum user Veracious, the original post can be found here on the official Magento forum.
First you need to download an SSH client, I used PuTTY
log into your hosting account through SSH and go to the root of your website and execute these commands
Firstly you need to navigate to where your Magento index.php is stored, which if it is in the root you get to by entering
cd /home/example/public_html
Now you can begin upgrading Magento
wget http://www.magentocommerce.com/downloads/assets/1.5.0.1/magento-1.5.0.1.tar.gz
tar -zxvf magento-1.5.0.1.tar.gz
Skip the next 4 commands if you have your magento install in a folder called Magento:
cd magento
cp -rf * /home/example/public_html
cd ../
rm -rf magento
rm magento-1.5.0.1.tar.gz
If all went well you will now be sat happy with the end result of your Magento Store running on the update version 1.5