This is the most comprehensive description of the Magento 1.3.x, 1.4.x, 1.5.x, 1.6.x and 1.7.x upgrade process.

Additionally It contains step-by-step instructions and troubleshooting information.

Prepare for Magento upgrade (this part of the article is for old 1.3.x, 1.4.x versions only)

Lets imaging that you have old Magento 1.3.x - 1.4.x store and you need to upgrade it to latest Magento 1.7.x version. First of all it is highly recommended to backup your live store files/database and disable Magento compiler cache.

Next you need to get SSH access from your hosting provider and connect to your server via SSH protocol (using Linux command shell or Putty SSH client for Windows).

After connection via SSH, go to your store folder and execute these commands:

1 - Change permissions on lib/pear folder to writable (recursively):