Before you make any changes to your existing site, please make sure you make full backup of your site using Akeeba Backup or manual backup.
The most probable reason behind upgrading your site to 2.x.x is that you want to upgrade your Joomla 1.5 to 2.5 or higher(or maybe you want to use the latest version).
Ok, now lets get down to business.
Create media directory for new Virtuemart at /images/stories/virtuemart/ using an FTP program or via your Web hosting Control Panel. Now move/copy all your Virtuemart images located at /components/com_virtuemart/shop_image/ to the one created earlier that is , /images/stories/virtuemart/ .
Now rename or delete your Virtuemart component folder located at /components/com_virtuemart/ something like /components/com_virtuemart-old .
Disable all modules and plugins that are related to Virtuemart using the Extension> Modules Manager and Extension>Plugin Manager respectively.
If you are running Joomla 1.5 then please download Virtuemart.2.0.20b from Virtuemart's website(this is the version supported for Joomla 1.5). You will get two files in the download, one is the component and the other is the All in One Package which has all the modules and plugins.
Now install the downloaded component file via Joomla Extensions> Install/Uninstall.
If all goes well you will see a success message.. If anything goes wrong, uninstall Virtuemart and then install the new 2.x.x component again. It should work.
Now your shop is upgraded to VM2 but you will find that all your datas are lost. NO, we haven't migrated the datas yet.
To Migrate all your shop datas, go to your new Virtuemart Component and then on the left panel select Configuration > Configuration and then enable the option called "Enable database Update tools " located under the Shop tab
After enabling the Database Update tools, now navigate to Tools > Tools & Migration on the left panel. You will be presented with a screen like
Now select Migration tab and then you will be presented with options to choose the type of data migration. You can just migrate Products, Orders, Categories etc only..
If you want to migrate everything select Choose a task for migration > Everything. Let the other option remain as it is or select any of the options to suit your need..
Now press the button called Start Migration, your migration time duration will depend on the number of products/categories you have. If your Migration is smooth, you will see a message from Virtuemart telling you that the migration has been completed. If this fail, please migrate each datas individually, like migrating products only and then categories, so on...
That's it. If you have any problem migrating your shop, do drop a comment we will try to help you out.