Upgrading Virtuemart for Joomla 1.5.26 is a pretty straightforward process and doesn't take too much time.
Here's a step-by-step guide:
Download the latest version of Virtuemart (for Joomla 1.5.26) from the official website (http://www.virtuemart.net/downloads);
Place the downloaded file into your Joomla installation folder;
Using FTP client, access your Joomla directory and separate the zip file of the Virtuemart package in the components folder;
Go to Extensions manager in your Joomla administrator panel, find the list of installed components and select the Virtuemart version you placed in the content folder;
Click the “Upgrade” button, enter your FTP information for the secure access and click the “Upgrade” button again;
Once the upgrade is complete, use the VirtueMart ‘Change Version’ button to ensure that the version installed properly. You will be asked to enter your VirtueMart license code;
Go to VirtueMart settings page to review settings and make changes if required;
Change the VirtueMart configuration to enable the upgrade process and if everything checks out, click on "save configuration";
Updated Virtuemart is ready now, you can go to the frontend in order to test out the newly installed version.
I hope this guide helps you upgrade Virtuemart for Joomla 1.5.26. If you encounter any difficulties during the process, please feel free to ask for help in this forum. Good luck!