As for the version, 11.20, since it's the officially supported version of QlikView right now. Version 11 was merged with 11.20 and they no longer coexist, only remaining 11.20.
As for the procedure, there is a nice PDF guide on the steps you have to follow to successfully upgrade in this document.
Basically, back up the C:\ProgramData\QlikTech folder or its equivalent if you have not done the default installation, as well as of course any QVW and QVD file you are using.
Hope that helps.