While it is not recommended, there are occasions that you may want to disable the secure boot on your machine, for example, for testing
To do so, first install
sudo apt install mokutil
sudo mokutil --disable-validationwhich will ask you to enter a password that will be used later for validation.
Reboot your machine and it should enter a blue screen.
change secure boot stage and type the password you entered above.
After that, you can boot into your OS and the secure boot should already be disabled.