Although all features are enable on the open source version of XenServer itself, some features are disabled when it comes to managing them using Citrix tools like XenCenter and one of the affected feature is the installation of patches. You will notice that the option to apply patches on your servers are greyed out.
You can however apply the patches manually. All your need to do is the get to the server console and run a few commands.
Before that, transfer the updates to the server or pool master using transfer tools like Filezilla and then run the following command
xe patch-upload file-name=
The command above will provide you with a UUID which you will use when you run the next command, if you are applying on a pool
xe patch-pool-apply uuid=
If you are applying for a single server, then you will need the host UUID which you can find through
XenCenter. The command will then be
You can monitor the updates progress in XenCenter. If the server is showing the arrow down icon, the updates are being applied will turn back to a green circle icon once it is done.
For can find more details here