On my Acer Aspire A515-52-51EL Secure Boot is enabled by default.
When accessing the BIOS/UEFI by pressing the F2 key during boot, the Secure Boot option is [Enabled] and greyed out so I couldn’t change it.
In order to change it you need to temporarily set a supervisor password in the BIOS.
Go to the Security page and navigate to Set Supervisor Password.
Press enter. This will show a prompt to set a supervisor password.
Important: DO NOT use a hard-to-remember password here! I recommend using abc123 as a password and writing it down just in case. If you forget your supervisor password you WILL BRICK YOUR MACHINE! This password will only be used temporarily so you DO NOT want to use a secure password!
Enter the password, press return, enter the password again and press return again. On the top of the page, Supervisor password is should now read Set.
Now go to the Boot tab where you can now disable Secure Boot.
Warning: DO NOT SKIP THE NEXT STEP!
Now we will unset the supervisor password. Go to the Security tab again, go to Set Supervisor Password and press return.
Enter the password you used before (e.g. abc123), then press enter. In the Enter new password and Confirm New Password fields, DO NOT ENTER ANY PASSWORD! Just press return two times.
Double-check that Supervisor password is now read Clear. This means that we un-set our supervisor password.
Important: If Supervisor password is does not read Clear, repeat the last step to ensure there is no supervisor password set.
Now go to Exit, select Exit Saving Changes and press return. Select Yes and press Enter.
Even though we have unset the supervisor password, secure boot will stay disabled.