By default, your grub menu should show up every time you boot.
If it doesn’t, boot your PC and do: sudo nano /etc/default/grub
You need these lines: GRUB_TIMEOUT=10 GRUB_TIMEOUT_STYLE=menu
Every line starting with: GRUB_HIDDEN_TIMEOUT
should be commented out like so: #GRUB_HIDDEN_TIMEOUT
Then run sudo update-grub and reboot.
What this does:
sets a countdown of 10 seconds before grub boots the kernel
tells grub to show the boot menu during that countdown
doesn’t use a hidden countdown that waits for a button press to show the menu
In the grub menu, select advanced options and there you should be able to select an older kernel to boot.