Stop your QuickBuild server. 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 temp directory.
Open a command line window, and switch to bin sub directory under extracted directory above. Run command upgrade.(bat|sh) <install directory of current QuickBuild> to upgrade QuickBuild in-place. The upgrade script will update target directory to new version so that QuickBuild installation directory remains unchanged.
After the upgrade, start your current QuickBuild instance. Agents will be auto-updated after server startup. It will take a while for them to appear in the build grid.
Verify server and agent installation by running some builds.