Hi @nisenycyeq226 ,
Kernel setting is a 
, so only do the actions you understand and do backups, ok? 
Kernel already present
Maybe you already got a 4.19 kernel, but it isn’t the current kernel.
Look from dom0 :
- /boot content
- /boot/efi/EFI/qubes/xen.cfg
If a 4.19 kernel is present, carefully read changing-default-kernel (change the default kernel entry).
Installing a specific kernel version
Again take care of what you do ().
The how-to-downgrade-a-specific-package documentation say you can use the package-version form.
How to get the list of the kernels ? See dnf search advanced options, or look the qubes 4.0 dom0 repository.