Applies to: Fedora 38 and higher, Centos 7 and higher

See which kernel version you will boot into on next boot:

root@mysystem:~# grubby --default-kernel
/boot/vmlinuz-6.9.4-100.fc39.x86_64

Review all kernels you have on your system

root@mysystem:~# grubby --info=ALL
index=0
kernel="/boot/vmlinuz-6.9.4-200.fc40.x86_64"
args="ro rhgb quiet selinux=0"
root="UUID=18d4b2e8-d77d-4184-8b5e-93f4f5885699"
initrd="/boot/initramfs-6.9.4-200.fc40.x86_64.img"
title="Fedora Linux (6.9.4-200.fc40.x86_64) 40 (Server Edition)"
id="3e64ec901c1c4472971f4e44f09e790d-6.9.4-200.fc40.x86_64"
index=1
kernel="/boot/vmlinuz-6.9.4-100.fc39.x86_64"
args="ro rhgb quiet selinux=0"
root="UUID=18d4b2e8-d77d-4184-8b5e-93f4f5885699"
initrd="/boot/initramfs-6.9.4-100.fc39.x86_64.img"
title="Fedora Linux (6.9.4-100.fc39.x86_64) 39 (Server Edition)"
id="3e64ec901c1c4472971f4e44f09e790d-6.9.4-100.fc39.x86_64"
index=2
kernel="/boot/vmlinuz-6.8.9-100.fc38.x86_64"
args="ro rhgb quiet selinux=0"
root="UUID=18d4b2e8-d77d-4184-8b5e-93f4f5885699"
initrd="/boot/initramfs-6.8.9-100.fc38.x86_64.img"
title="Fedora Linux (6.8.9-100.fc38.x86_64) 38 (Server Edition)"
id="3e64ec901c1c4472971f4e44f09e790d-6.8.9-100.fc38.x86_64"
index=3
kernel="/boot/vmlinuz-0-rescue-3e64ec901c1c4472971f4e44f09e790d"
args="ro rhgb quiet selinux=0"
root="UUID=18d4b2e8-d77d-4184-8b5e-93f4f5885699"
initrd="/boot/initramfs-0-rescue-3e64ec901c1c4472971f4e44f09e790d.img"
title="Fedora Linux (0-rescue-3e64ec901c1c4472971f4e44f09e790d) 38 (Server Edition)"
id="3e64ec901c1c4472971f4e44f09e790d-0-rescue"

I want my system to by default boot to 6.9.4-200.fc40.x86_64

root@mysystem:~# grubby --set-default /boot/vmlinuz-6.9.4-200.fc40.x86_64
The default is 3e64ec901c1c4472971f4e44f09e790d-6.9.4-200.fc40.x86_64.conf with index 0 and kernel /boot/vmlinuz-6.9.4-200.fc40.x86_64

Next time it will boot to version 6.9.4-200