Citrix XenApp and XenDesktop can be upgraded, but must be done in a particular order to avoid downtime.
Prepare - run site checks, backup databases, check Subscription Advantage date, close apps and console and check permissions.
Upgrade License server
Upgrade Provisioning Services servers and Target Device software
Upgrade half the Delivery Controllers
Upgrade XenApp/XenDesktop databases
Upgrade remaining Delivery Controllers
Upgrade Virtual Delivery Agents (VDA)
Update machine catalog and delivery groups
The following article goes through the steps of upgrading the Citrix StoreFront component. It is a very straight forward process. All StoreFront servers that are part of the same Server Group must be upgraded for it to be successful.
Insert the Citrix XenApp/XenDesktop 7.13 installation media and the Autorun program should run. Click on Upgrade Studio and Server Components.
Accept the terms of the license agreement and click Next.
Select I'm ready to continue and click Next.
If you want diagnostics data to be uploaded to Citrix Insight Services then select I want to participate in Call Home otherwise select I do not want to participate in Call Home and click Next.
Click Finish. Repeat this steps for each StoreFront server in the Server Group to complete the upgrade.