Since XenServer 6.2 came out, things have changed for the Citrix developed virtualisation environment. Specifically, the free licence no longer applies - you get a fully featured system out of the box and licensing is applied per socket. What this means in a practical sense for keeping your server updated is that using XenCenter to update it is not an option unless you've licensed it.
If you're like me and running XenServer in a quasi test / virtual environment, you won't have paid for a licence (as much as you'd like to). Therefore updating your VM hosts is a bit different and you have to use the command line. Now, it's important to note a couple of things that the XenServer FAQs don't necessarily specify. You have to use the xe command line program to upload and apply the patches. Do this in the following way:
Open up a cmd process (Windows+r, then type cmd and hit enter).
Go to the correct directory: cd C:\Program Files (x86)\Citrix\XenCenter and hit enter.
In this folder is the xe.exe application we'll need. The next thing to do is download the patches from Citrix. Usually they'll land in your Downloads folder. Extract the files from within and note where you've put them - typically it ends up being c:\users\ryv\\Downloads\XS62E002\ for example
You have to use xe.exe to upload and apply these patches to your server - send it to the Pool Master using either it's IP or hostname (if you have DNS set up correctly).
The syntax is:
xe patch-upload -s <hostname / IP> -u root -pw <password> file-name=<path to file>\XS62E002.xsupdate
In real life this command might look like this:
xe patch-upload -s 10.0.0.100 -u root -pw s3cret123 file-name=c:\users\ryv\Downloads\XS62E002\XS62E002.xsupdate
Once the file is uploaded, it gives you a UUID of the hotfix. It might look like this: 59128f15-92cd-4dd9-8fbe-a0115d1b07b4
Make a note of this - we'll need it in a tick.
To apply the hotfix to your hosts in the pool, the syntax looks like this:
xe -s <hostname / IP> -u root -pw <password> patch-pool-apply uuid=<hotfix UUID> and then hit enter.
In real life it might look like this:
xe -s 10.0.0.100 -u root -pw s3cret123 patch-pool-apply uuid=59128f15-92cd-4dd9-8fbe-a0115d1b07b4
You'll really need to reboot the hosts after this. Verifying the application is easy - go to XenCentre and choose your Pool, then click the General Tab and expand Updates. Note that the update is "Fully Applied" and you're done!