I found something which works :
first install the kernel
echo 'deb http://ftp.debian.org/debian/ stretch-backports main' | tee -a /etc/apt/sources.list.d/backports.list
apt-get update
apt-get -t stretch-backports install linux-image-amd64
Edit this file /etc/default/grub and replace GRUB_DEFAULT=0 by 1 or 2 (select the number of installed kernel, 0 for the default) for me i installed only on more. So kernel number is 1.
More detail here: https://github.com/dannyti/sboxsetup/wiki/Changing-OVH-Server-Kernel
GRUB_DEFAULT=1
run
update-grub
then reboot
reboot