Upgrade your existing citrix site
Upgrading changes deployments to the newest component versions without having to set up new machines or Sites. This is known as an in-place upgrade. You can upgrade to the current version from:
XenDesktop 5.6 *
XenApp/XenDesktop 7.6 LTSR
XenApp/XenDesktop 7.15 LTSR
* To upgrade from XenDesktop 5.6, first upgrade to 7.6 LTSR (with the latest CU), and then upgrade to 7.15 LTSR (with the latest CU).
You can also upgrade a XenApp 6.5 worker server to a current VDA for Windows Server OS. This is a supplementary activity to migrating XenApp 6.5. See Upgrade a XenApp 6.5 worker to a new VDA for Windows Server OS
Run the installer on the machines where the core components and VDAs are installed. The software determines if an upgrade is available and installs the newer version.
Use the newly upgraded Studio to upgrade the database and the Site.
For details, see Upgrade a deployment.
For information about installing Controller hotfixes, see CTX201988.
Migrating moves data from an earlier deployment to a newer version. You can migrate from XenApp 6.x to XenApp 7.6. Migrating includes installing current components and creating a new Site, exporting data from the older farm, and then importing the data to the new Site.
For information about architecture, component, and feature changes that were introduced with the 7.x releases, see Changes in 7.x.
For more information about migrating, see Migrate XenApp 6.x.
Please visit the below link for instruction to upgrade existing deployment