After activating the debian buster backports repo please update your package list with
sudo apt update
You can install the packages.
linux-image-5.8.0-0.bpo.2-amd64
&
linux-headers-5.8.0-0.bpo.2-amd64
For Kernel 5.8.10
The terminal command
sudo apt install linux-image-5.8.0-0.bpo.2-amd64 linux-headers-5.8.0-0.bpo.2-amd64
should suffice.
After that please reboot.
Note: If you have virtualbox installed make sure to update it from backports otherwise the dkms virtualbox package might display an error and virtualbox will not be usable with the newer kernel.
You can use sudo apt install virtualbox-dkms with backports activated to update it.