VirtueMart 1.1.3 is a recommended security update, which provides fixes for some security issues, other enhancements and fixes a lot of minor bugs (see the changelog for full details).
Full installation packages and patch packages are available on the download page.
To update to version 1.1.3, login to your VirtueMart administration area and choose "Check for Updates" from the "Admin" menu, then click the "Check now!" button and follow the instructions there. (Or, you can download the 1.1.3 patch package directly , save it to your local hard drive, and choose to "Upload a Patch Package" from the "Upload a Patch" tab.)
If you are using VirtueMart 1.1.0, you must first update to VirtueMart 1.1.1 before updating to VirtueMart 1.1.3.