Stop QuickBuild server of previous version, you do not need to stop agents since they will be auto-upgraded after server is upgraded.
Extract installation package of new version to a new directory.
Open a command line window, and switch to the bin sub directory under the new version 's installation directory. Run command migrate.(bat|sh) <install directory of previous QuickBuild version> to migrate from previous QuickBuild version.
For windows platform, NT service will be installed automatically for the new QuickBuild server if old QuickBuild server is configured to run as NT service. However the new service runs with local system account initially and the "interact with desktop" option is disabled. You will need to change this manually after migration if necessary.
Start QuickBuild server in new installation directory and agents will be auto-updated. It will take a while for them to appear in the build grid.
Verify server and agent installation by running some builds.