This instruction in simplified version of [official Fedora instruction](https://fedoraproject.org/wiki/Upgrading_Fedora_using_yum). Note that only "yum" method will work in Qubes template VM (if you are curious why: mostly because template VM does not have own bootloader).
If you have installed a lot of software in your template, it may happen that you don't have enough disk space for upgrade. Yum will tell you this just after confirming the operation (before it change anything to your system). In that case you need to provide some "external" place for yum cache (uses about 2GB during upgrade). For example attach virtual disk stored in some file. Prepare the file in dom0:
fstrim, nor "discard" mount option do not work on template root fs, so when some file is removed in the template, space isn't freed in dom0. This means that template will use about twice a space that is really need after upgrade.
You can compact root.img in the "old way", you'll need about 15GB (template's max size + really used space there) in dom0 for this operation: Start the template, fill all the free space with zeros, for example with:
