Please how should I install new realtime new kernel for linuxcnc 2.8.4

You should try the new ISO from the downloads page, it has a new RT kernel that should be a bit better for latency. You can do that without installing, just boot the PC from USB and test the latency.
If it still does not work, on your current 2.8.4 :
sudo apt update
sudo apt upgrade
if it fails
sudo apt install -f
then
sudo apt upgrade
and if all else fails,
linuxcnc.org/dists/buster/2.8-rt/binary-...xcnc_2.8.4_amd64.deb
download, double click and it should start gdebi, if it does not
sudo apt install gdebi
then double click again.

In a terminal
uname -a
?

Result of
uname -a
???
-
sudo apt purge linuxcnc-uspace
then install deb package.

Boot the PC, press and hold SHIFT, when grub menu appears, see if there is something with RTAI in the name, choose that and press enter.