There isn't much to tell about such an upgrade. Most of the time, I did it in-site without removing the old installation and with no adverse effects.
However, QlikTech advises to remove the old 11.0 before installing the new one. See the 11.20SR6 Release Notes. So you should make a copy of C:\ProgramData\QlikTech before deleting v11.0
If you have ODBC connections to SQL Server and/or other MS products, and you experience intermittent reload failures after the upgrade, you may want to check out this thread Error after upgrading to version 11.20 SR2
In that case, the following document may be of help. It's not about 11.2 or migrating from 11 to 11.2 (that's not really a migration, just a simple upgrade) but it contains a nice overview of the process of moving various QlikView services to other machines. See Section-5: Re-locating QlikView Services.