setting older kernel version as default

The simplest solution might be to enable Grub's GRUB_SAVEDEFAULT feature. Try this:

0. Open the Terminal
1. Edit the /etc/default/grub file: sudo vi /etc/default/grub
2. Change GRUB_DEFAULT=0 to GRUB_DEFAULT=saved
3. Add GRUB_SAVEDEFAULT=true
4. Save the file (Esc⇢:⇢W⇢X
5. Update Grub with: sudo update-grub

These settings will ensure that your system boots with the last kernel you chose during boot.

Now reboot, hold the shift key when your system comes back up. You should then see the Grub menu. Choose the Kernel that works for you. That will now be "default".

Hope this gives you what you need.

If you'd like to always boot specific version by default you should

1. Edit /etc/default/grub file. Find GRUB_DEFAULT and specify path to the grub menu entry (something like GRUB_DEFAULT='Advanced options for Ubuntu>Ubuntu, with Linux 5.4.0-48-generic')
2. Run update-grub.

Note 1. You can find grub menu entries in /boot/grub/grub.cfg.
Note 2. I recommend to make a copy of /etc/default/grub before modification.

You can also check this link - I've created a note for myself some time ago.