Default OS after Fedora installation

First check current kernels in the system:
# sudo grubby --info=ALL | grep -E "^kernel|^index"
index=0
kernel="/boot/vmlinuz-6.XYZ
index=1
kernel="/boot/vmlinuz-6.ABC
index=2
kernel="/boot/vmlinuz-6.PQR
index=3
kernel="/boot/vmlinuz-0-rescue-ABCCD"

Set the default index:
# sudo grubby --set-default-index=0 
The default is /boot/loader/entries/86b961286bda43e580ff4adfd8da7d0e-6.11.10-200.fc40.x86_64.conf with index 0 and kernel /boot/vmlinuz-6.XYZ

Cross check the set kernel:
root@fedora:/home/saran# sudo grubby --default-kernel 
/boot/vmlinuz-6.XYZ