Defining new default kernel has some known bugs with btrfs (if you search in the forum you'll find several topics about it).
So, you should edit /etc/default/grub and change GRUB_DEFAULT=0 (which is the first line you see in the GRUB) to e.g. GRUB_DEFAULT=1 (second line) or whatever else, also in sub-menu you can use e.g. GRUB_DEFAULT="1>2" (third line in second line submenu).
You should also comment out GRUB_SAVEDEFAULT, e.g.
#GRUB_SAVEDEFAULT=true
Pay attention, sometimes some parameters (e.g. GRUB_DEFAULT) might appear more than once in the grub file).
Then update-grub of course.