The easiest way for the current running kernel is:
update-initramfs -u -k `uname -r`
You could change -k to a specific kernel name if for some reason the current is not running (eg. if you are chrooted or in recovery mode).
If you want to update all kernels then use "-k all"
update-initramfs -k all -u
update-initramfs: Generating /boot/initrd.img-5.4.0-162-generic........